Fix for template compilation on Rails 4.0

This fixes template compilation on Rails 4.0.0.rc1.
I basically had a look at:

   https://github.com/metaskills/less-rails/blob/master/lib/less/rails/railtie.rb

and used their method to register the engine with sprockets.

I have not tested this with older Rails versions.

Issue: https://github.com/emberjs/ember-rails/issues/185