Remove Adapter autoloads in favor of require

Adapters must be eager loaded to ensure they are defined
before they are used as namespacing.

cf6a074a1c (diff-41f2b3509d33e1c65bb70ee0ec7a2eea)

lib/active_model/serializer/associations.rb

@@ -15,6 +15,7 @@ module ActiveModel
           attr_accessor :_reflections
         end
 
+        extend ActiveSupport::Autoload
         autoload :Association
         autoload :Reflection
         autoload :SingularReflection