AC::Caching::Sweeper is not properly loaded

Probably since the renaming from active_record-observers to rails-observers, AC::Caching::Sweeper is not properly loaded if AC::Base was loaded before loading AR::Base, because the Sweeper class is defined like [this](https://github.com/rails/rails-observers/blob/0711082cdc81c15c04d348a59ac181ed0b8b4867/lib/rails/observers/action_controller/caching/sweeping.rb#L51).

This situation would happen very naturally because AC initializer would be placed prior to AR initializer when tsorted by &:name, and so the problem can actually easily reproduced on a vanilla Rails 4.1 app that calls `cache_sweeper`.