Revert "First part of a fix for #208."

This reverts commit 145b6d499d.

This fix introduced an incompatibility with Rails 3.0.

We'll have to fix it in a different way for older Rails.

lib/active_model_serializers.rb

@@ -20,9 +20,7 @@ if defined?(Rails)
 
       initializer "include_routes.active_model_serializer" do |app|
         ActiveSupport.on_load(:active_model_serializers) do
-          include AbstractController::UrlFor
-          extend ::AbstractController::Railties::RoutesHelpers.with(app.routes)
-          include app.routes.mounted_helpers
+          include app.routes.url_helpers
         end
       end