Rename dependencies.rake to cache_digests.rake

As the tasks are related to cache_digests and
as they are already namespaced under cache_digests,
renaming to cache_digests.rake makes it to know
where to find these tasks.

actionview/lib/action_view/railtie.rb

@@ -59,7 +59,7 @@ class Railtie < Rails::Railtie # :nodoc:
 
     rake_tasks do |app|
       unless app.config.api_only
-        load "action_view/tasks/dependencies.rake"
+        load "action_view/tasks/cache_digests.rake"
       end
     end
   end