Require Ruby 2.5 for Rails 6.

Generally followed the pattern for https://github.com/rails/rails/pull/32034

* Removes needless CI configs for 2.4
* Targets 2.5 in rubocop
* Updates existing CHANGELOG entries for fewer merge conflicts
* Removes Hash#slice extension as that's inlined on Ruby 2.5.
* Removes the need for send on define_method in MethodCallAssertions.

4 files changed, 9 insertions, 34 deletions

diff --git a/activesupport/CHANGELOG.md b/activesupport/CHANGELOG.md
index ba8aaf47f9..e678f48244 100644
--- a/activesupport/CHANGELOG.md
+++ b/activesupport/CHANGELOG.md
@@ -282,9 +282,9 @@
 
     *Jeremy Daer*
 
-*   Rails 6 requires Ruby 2.4.1 or newer.
+*   Rails 6 requires Ruby 2.5.0 or newer.
 
-    *Jeremy Daer*
+    *Jeremy Daer*, *Kasper Timm Hansen*
 
 *   Adds parallel testing to Rails.
 
diff --git a/activesupport/activesupport.gemspec b/activesupport/activesupport.gemspec
index 448a2eeebb..bdd7bc70a0 100644
--- a/activesupport/activesupport.gemspec
+++ b/activesupport/activesupport.gemspec
@@ -9,7 +9,7 @@ Gem::Specification.new do |s|
   s.summary     = "A toolkit of support libraries and Ruby core extensions extracted from the Rails framework."
   s.description = "A toolkit of support libraries and Ruby core extensions extracted from the Rails framework. Rich support for multibyte strings, internationalization, time zones, and testing."
 
-  s.required_ruby_version = ">= 2.4.1"
+  s.required_ruby_version = ">= 2.5.0"
 
   s.license = "MIT"
 
diff --git a/activesupport/lib/active_support/core_ext/hash/slice.rb b/activesupport/lib/active_support/core_ext/hash/slice.rb
index e07a5498ff..3d0f8a1e62 100644
--- a/activesupport/lib/active_support/core_ext/hash/slice.rb
+++ b/activesupport/lib/active_support/core_ext/hash/slice.rb
@@ -1,29 +1,6 @@
 # frozen_string_literal: true
 
 class Hash
-  # Slices a hash to include only the given keys. Returns a hash containing
-  # the given keys.
-  #
-  #   { a: 1, b: 2, c: 3, d: 4 }.slice(:a, :b)
-  #   # => {:a=>1, :b=>2}
-  #
-  # This is useful for limiting an options hash to valid keys before
-  # passing to a method:
-  #
-  #   def search(criteria = {})
-  #     criteria.assert_valid_keys(:mass, :velocity, :time)
-  #   end
-  #
-  #   search(options.slice(:mass, :velocity, :time))
-  #
-  # If you have an array of keys you want to limit to, you should splat them:
-  #
-  #   valid_keys = [:mass, :velocity, :time]
-  #   search(options.slice(*valid_keys))
-  def slice(*keys)
-    keys.each_with_object(Hash.new) { |k, hash| hash[k] = self[k] if has_key?(k) }
-  end unless method_defined?(:slice)
-
   # Replaces the hash with only the given keys.
   # Returns a hash containing the removed key/value pairs.
   #
diff --git a/activesupport/lib/active_support/testing/method_call_assertions.rb b/activesupport/lib/active_support/testing/method_call_assertions.rb
index eba41aa907..03c38be481 100644
--- a/activesupport/lib/active_support/testing/method_call_assertions.rb
+++ b/activesupport/lib/active_support/testing/method_call_assertions.rb
@@ -35,18 +35,16 @@ module ActiveSupport
           assert_called(object, method_name, message, times: 0, &block)
         end
 
-        # TODO: No need to resort to #send once support for Ruby 2.4 is
-        # dropped.
         def assert_called_on_instance_of(klass, method_name, message = nil, times: 1, returns: nil)
           times_called = 0
-          klass.send(:define_method, "stubbed_#{method_name}") do |*|
+          klass.define_method("stubbed_#{method_name}") do |*|
             times_called += 1
 
             returns
           end
 
-          klass.send(:alias_method, "original_#{method_name}", method_name)
-          klass.send(:alias_method, method_name, "stubbed_#{method_name}")
+          klass.alias_method "original_#{method_name}", method_name
+          klass.alias_method method_name, "stubbed_#{method_name}"
 
           yield
 
@@ -55,9 +53,9 @@ module ActiveSupport
 
           assert_equal times, times_called, error
         ensure
-          klass.send(:alias_method, method_name, "original_#{method_name}")
-          klass.send(:undef_method, "original_#{method_name}")
-          klass.send(:undef_method, "stubbed_#{method_name}")
+          klass.alias_method method_name, "original_#{method_name}"
+          klass.undef_method "original_#{method_name}"
+          klass.undef_method "stubbed_#{method_name}"
         end
 
         def assert_not_called_on_instance_of(klass, method_name, message = nil, &block)