Using the relation subclass created by Rails 4, instead of creating a subclass inside friendly_id

This is a tiny patch to use friendly_id with Rails 4, this solves the "synchronize" exception mentioned in #357. We are using this patch in a OS project here (http://github.com/ideiasaleatorias/agendatech / http://agendatech.com.br/) on the branch rails4.

I can enhance the solution if you feel it's necessary, but i'm having trouble with `bundle install` in this project. Bundler appears to "stay forever" searching for the gems, and apparently it don't solve the dependencies graph.