Suppress `warning: BigDecimal.new is deprecated` in activerecord

`BigDecimal.new` has been deprecated in BigDecimal 1.3.3
 which will be a default for Ruby 2.5.

Refer https://github.com/ruby/bigdecimal/commit/533737338db915b00dc7168c3602e4b462b23503

```
$ cd rails/activerecord/
$ git grep -l BigDecimal.new | grep \.rb | xargs sed -i -e "s/BigDecimal.new/BigDecimal/g"
```

- Changes made only to Active Record. Will apply the same change to
other module once this commit is merged.

- The following deprecation has not been addressed because it has been
reported at `ActiveRecord::Result.new`. `ActiveRecord::Result.ancestors`
did not show `BigDecimal`.

* Not addressed

```ruby
/path/to/rails/activerecord/lib/active_record/connection_adapters/mysql/database_statements.rb:34:
warning: BigDecimal.new is deprecated
```

* database_statements.rb:34

```ruby
ActiveRecord::Result.new(result.fields, result.to_a) if result
```

* ActiveRecord::Result.ancestors

```ruby
[ActiveRecord::Result,
 Enumerable,
 ActiveSupport::ToJsonWithActiveSupportEncoder,
 Object,
 Metaclass::ObjectMethods,
 Mocha::ObjectMethods,
 PP::ObjectMixin,
 ActiveSupport::Dependencies::Loadable,
 ActiveSupport::Tryable,
 JSON::Ext::Generator::GeneratorMethods::Object,
 Kernel,
 BasicObject]
```

This commit has been tested with these Ruby and BigDecimal versions

- ruby 2.5 and bigdecimal 1.3.3

```
$ ruby -v
ruby 2.5.0dev (2017-12-14 trunk 61217) [x86_64-linux]
$ gem list |grep bigdecimal
bigdecimal (default: 1.3.3, default: 1.3.2)
```

- ruby 2.4 and bigdecimal 1.3.0

```
$ ruby -v
ruby 2.4.2p198 (2017-09-14 revision 59899) [x86_64-linux-gnu]
$ gem list |grep bigdecimal
bigdecimal (default: 1.3.0)
```

- ruby 2.3 and bigdecimal 1.2.8

```
$ ruby -v
ruby 2.3.5p376 (2017-09-14 revision 59905) [x86_64-linux]
$ gem list |grep -i bigdecimal
bigdecimal (1.2.8)
```

- ruby 2.2 and bigdecimal 1.2.6
```
$ ruby -v
ruby 2.2.8p477 (2017-09-14 revision 59906) [x86_64-linux]
$ gem list |grep bigdecimal
bigdecimal (1.2.6)
```

1 files changed, 7 insertions, 7 deletions

diff --git a/activerecord/test/cases/adapters/postgresql/range_test.rb b/activerecord/test/cases/adapters/postgresql/range_test.rb
index a75fdef698..813a8721a2 100644
--- a/activerecord/test/cases/adapters/postgresql/range_test.rb
+++ b/activerecord/test/cases/adapters/postgresql/range_test.rb
@@ -134,10 +134,10 @@ _SQL
     end
 
     def test_numrange_values
-      assert_equal BigDecimal.new("0.1")..BigDecimal.new("0.2"), @first_range.num_range
-      assert_equal BigDecimal.new("0.1")...BigDecimal.new("0.2"), @second_range.num_range
-      assert_equal BigDecimal.new("0.1")...BigDecimal.new("Infinity"), @third_range.num_range
-      assert_equal BigDecimal.new("-Infinity")...BigDecimal.new("Infinity"), @fourth_range.num_range
+      assert_equal BigDecimal("0.1")..BigDecimal("0.2"), @first_range.num_range
+      assert_equal BigDecimal("0.1")...BigDecimal("0.2"), @second_range.num_range
+      assert_equal BigDecimal("0.1")...BigDecimal("Infinity"), @third_range.num_range
+      assert_equal BigDecimal("-Infinity")...BigDecimal("Infinity"), @fourth_range.num_range
       assert_nil @empty_range.num_range
     end
 
@@ -285,14 +285,14 @@ _SQL
 
     def test_create_numrange
       assert_equal_round_trip(@new_range, :num_range,
-                              BigDecimal.new("0.5")...BigDecimal.new("1"))
+                              BigDecimal("0.5")...BigDecimal("1"))
     end
 
     def test_update_numrange
       assert_equal_round_trip(@first_range, :num_range,
-                              BigDecimal.new("0.5")...BigDecimal.new("1"))
+                              BigDecimal("0.5")...BigDecimal("1"))
       assert_nil_round_trip(@first_range, :num_range,
-                            BigDecimal.new("0.5")...BigDecimal.new("0.5"))
+                            BigDecimal("0.5")...BigDecimal("0.5"))
     end
 
     def test_create_daterange