Load AC::Chaching::Sweeper if AC is defined when loading AR Observer

Without this, AC::Cashing::Sweeper could never be loaded if Rails initializer
loads AC before loading AR, which actually is Rails' default behaviour.

lib/rails/observers/activerecord/observer.rb

@@ -123,3 +123,7 @@ def define_callbacks(klass)
       end
   end
 end
+
+if defined?(ActionController) and defined?(ActionController::Caching::Sweeping)
+  require 'rails/observers/action_controller/caching/sweeper'
+end