Tests for cache digests implementation for Rails 4.
Tracker that is being used to track dependencies for `extends` and `partial` for each files.

Reference: https://github.com/rails/rails/blob/master/actionpack/lib/action_view/dependency_tracker.rb#L26
Custom digester for each file and its dependencies.  Inherited from `ActionView::Digestor`.

Reference: https://github.com/rails/rails/blob/master/actionpack/lib/action_view/digestor.rb
Special `cache_key` is being used by rails 4 from the custom `Rabl::Digester`.
Inject into Rails 4 so that the `cache digests` works.
Minor updates on the comment.