Fix for migration active_storage migration

*ActiveRecord::StatementInvalid: PG::UndefinedTable: ERROR:  relation "active_storage_attachments" does not exist*

Updating to rails 6.0.0.beta3 if the command rake app:update is used a new migration file is generated:

```ruby
# db/migrate/add_foreign_key_constraint_to_active_storage_attachments_for_blob_id.active_storage.rb
class AddForeignKeyConstraintToActiveStorageAttachmentsForBlobId < ActiveRecord::Migration[6.0]
  def up
    unless foreign_key_exists?(:active_storage_attachments, column: :blob_id)
  		add_foreign_key :active_storage_attachments, :active_storage_blobs, column: :blob_id
    end
  end
end
```

**If the project does not have previously installed active storage** this mgiration causes an error:

    ActiveRecord::StatementInvalid: PG::UndefinedTable: ERROR:  relation "active_storage_attachments" does not exist

**Error output:**

    PG::UndefinedTable: ERROR:  relation "active_storage_attachments" does not exist
    activerecord-6.0.0.beta3/lib/active_record/connection_adapters/postgresql/database_statements.rb:90:in `async_exec'
    activerecord-6.0.0.beta3/lib/active_record/connection_adapters/postgresql/database_statements.rb:90:in `block (2 levels) in execute'
    activesupport-6.0.0.beta3/lib/active_support/dependencies/interlock.rb:48:in `block in permit_concurrent_loads'
    activesupport-6.0.0.beta3/lib/active_support/concurrency/share_lock.rb:187:in `yield_shares'
    activesupport-6.0.0.beta3/lib/active_support/dependencies/interlock.rb:47:in `permit_concurrent_loads'
    activerecord-6.0.0.beta3/lib/active_record/connection_adapters/postgresql/database_statements.rb:89:in `block in execute'
    activerecord-6.0.0.beta3/lib/active_record/connection_adapters/abstract_adapter.rb:643:in `block (2 levels) in log'
    activerecord-6.0.0.beta3/lib/active_record/connection_adapters/abstract_adapter.rb:642:in `block in log'
    activesupport-6.0.0.beta3/lib/active_support/notifications/instrumenter.rb:23:in `instrument'
    activerecord-6.0.0.beta3/lib/active_record/connection_adapters/abstract_adapter.rb:633:in `log'
    activerecord-6.0.0.beta3/lib/active_record/connection_adapters/postgresql/database_statements.rb:88:in `execute'
    activerecord-6.0.0.beta3/lib/active_record/connection_adapters/abstract/schema_statements.rb:977:in `add_foreign_key'
    activerecord-6.0.0.beta3/lib/active_record/migration.rb:875:in `block in method_missing'
    activerecord-6.0.0.beta3/lib/active_record/migration.rb:843:in `block in say_with_time'
    activerecord-6.0.0.beta3/lib/active_record/migration.rb:843:in `say_with_time'
    activerecord-6.0.0.beta3/lib/active_record/migration.rb:864:in `method_missing'
    /db/migrate/20190315102607_add_foreign_key_constraint_to_active_storage_attachments_for_blob_id.active_storage.rb:5:in `up'
    activerecord-6.0.0.beta3/lib/active_record/migration.rb:816:in `exec_migration'
    activerecord-6.0.0.beta3/lib/active_record/migration.rb:797:in `block (2 levels) in migrate'
    activerecord-6.0.0.beta3/lib/active_record/migration.rb:796:in `block in migrate'
    activerecord-6.0.0.beta3/lib/active_record/connection_adapters/abstract/connection_pool.rb:416:in `with_connection'
    activerecord-6.0.0.beta3/lib/active_record/migration.rb:795:in `migrate'
    activerecord-6.0.0.beta3/lib/active_record/migration.rb:985:in `migrate'
    activerecord-6.0.0.beta3/lib/active_record/migration.rb:1293:in `block in execute_migration_in_transaction'
    activerecord-6.0.0.beta3/lib/active_record/migration.rb:1344:in `block in ddl_transaction'
    activerecord-6.0.0.beta3/lib/active_record/connection_adapters/abstract/database_statements.rb:265:in `block in transaction'
    activerecord-6.0.0.beta3/lib/active_record/connection_adapters/abstract/transaction.rb:272:in `block in within_new_transaction'
    activerecord-6.0.0.beta3/lib/active_record/connection_adapters/abstract/transaction.rb:270:in `within_new_transaction'
    activerecord-6.0.0.beta3/lib/active_record/connection_adapters/abstract/database_statements.rb:265:in `transaction'
    activerecord-6.0.0.beta3/lib/active_record/transactions.rb:212:in `transaction'
    activerecord-6.0.0.beta3/lib/active_record/migration.rb:1344:in `ddl_transaction'
    activerecord-6.0.0.beta3/lib/active_record/migration.rb:1292:in `execute_migration_in_transaction'
    activerecord-6.0.0.beta3/lib/active_record/migration.rb:1264:in `block in migrate_without_lock'
    activerecord-6.0.0.beta3/lib/active_record/migration.rb:1263:in `each'
    activerecord-6.0.0.beta3/lib/active_record/migration.rb:1263:in `migrate_without_lock'
    activerecord-6.0.0.beta3/lib/active_record/migration.rb:1211:in `block in migrate'
    activerecord-6.0.0.beta3/lib/active_record/migration.rb:1364:in `with_advisory_lock'
    activerecord-6.0.0.beta3/lib/active_record/migration.rb:1211:in `migrate'
    activerecord-6.0.0.beta3/lib/active_record/migration.rb:1044:in `up'
    activerecord-6.0.0.beta3/lib/active_record/migration.rb:1019:in `migrate'
    activerecord-6.0.0.beta3/lib/active_record/tasks/database_tasks.rb:191:in `migrate'
    activerecord-6.0.0.beta3/lib/active_record/railties/databases.rake:78:in `block (3 levels) in <main>'
    activerecord-6.0.0.beta3/lib/active_record/railties/databases.rake:76:in `each'
    activerecord-6.0.0.beta3/lib/active_record/railties/databases.rake:76:in `block (2 levels) in <main>'
    railties-6.0.0.beta3/lib/rails/commands/rake/rake_command.rb:23:in `block in perform'
    railties-6.0.0.beta3/lib/rails/commands/rake/rake_command.rb:20:in `perform'
    railties-6.0.0.beta3/lib/rails/command.rb:48:in `invoke'
    railties-6.0.0.beta3/lib/rails/commands.rb:18:in `<main>'
    bootsnap-1.4.1/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:21:in `require'
    bootsnap-1.4.1/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:21:in `block in require_with_bootsnap_lfi'
    bootsnap-1.4.1/lib/bootsnap/load_path_cache/loaded_features_index.rb:83:in `register'
    bootsnap-1.4.1/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:20:in `require_with_bootsnap_lfi'
    bootsnap-1.4.1/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:29:in `require'
    activesupport-6.0.0.beta3/lib/active_support/dependencies.rb:297:in `block in require'
    activesupport-6.0.0.beta3/lib/active_support/dependencies.rb:263:in `load_dependency'
    activesupport-6.0.0.beta3/lib/active_support/dependencies.rb:297:in `require'
    bin/rails:4:in `<main>'

1 files changed, 3 insertions, 1 deletions

diff --git a/activestorage/db/update_migrate/20180723000244_add_foreign_key_constraint_to_active_storage_attachments_for_blob_id.rb b/activestorage/db/update_migrate/20180723000244_add_foreign_key_constraint_to_active_storage_attachments_for_blob_id.rb
index 6830203cd6..844577d9c8 100644
--- a/activestorage/db/update_migrate/20180723000244_add_foreign_key_constraint_to_active_storage_attachments_for_blob_id.rb
+++ b/activestorage/db/update_migrate/20180723000244_add_foreign_key_constraint_to_active_storage_attachments_for_blob_id.rb
@@ -1,6 +1,8 @@
 class AddForeignKeyConstraintToActiveStorageAttachmentsForBlobId < ActiveRecord::Migration[6.0]
   def up
-    unless foreign_key_exists?(:active_storage_attachments, column: :blob_id)
+    return if foreign_key_exists?(:active_storage_attachments, column: :blob_id)
+
+    if table_exists?(:active_storage_attachments) && table_exists?(:active_storage_blobs)
       add_foreign_key :active_storage_attachments, :active_storage_blobs, column: :blob_id
     end
   end