Fixes an issue which doesn't update assets cache with rails 4

What was happening:
1. Rails.application.assets.register_preprocessor was always raising
TypeError: can't modify immutable index.
2. The error was rescued so looks it's done. But context.depend_on is
not registered so locale files changes didn't expires cache.

I found
https://github.com/rails/rails/issues/2211#issuecomment-1724180
and simply removed ActiveSupport.on_load (and the rescue) and everything
is working well.
Merge pull request #229 from masatomo/fix-locale-cache-issue

Fixes an issue which doesn't update assets cache with rails 4