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/adapter/fragment_cache.rb

@@ -1,4 +1,7 @@
-class ActiveModel::Serializer::Adapter::FragmentCache
+module ActiveModel
+  class Serializer
+    class Adapter
+      class FragmentCache
         attr_reader :serializer
 
         def initialize(adapter, serializer, options)
@@ -76,4 +79,7 @@ class ActiveModel::Serializer::Adapter::FragmentCache
         def to_valid_const_name(name)
           name.gsub('::', '_')
         end
+      end
+    end
+  end
 end