Favor ActiveSupport::TestCase over Minitest::Test

- Better minitest 4/5 support
- Better DSL
- Already available with no changes
- Consistent interface

test/serializers/meta_test.rb

@@ -2,7 +2,7 @@ require 'test_helper'
 
 module ActiveModel
   class Serializer
-    class MetaTest < Minitest::Test
+    class MetaTest < ActiveSupport::TestCase
       def setup
         @blog = Blog.new(id: 1,
                          name: 'AMS Hints',