fix: downgrade foundation-rails

There is an issue raised when compiling. Nothing addressed yet as it's
rather new but other people met this situation.

* https://stackoverflow.com/questions/25901261/heroku-zurb-foundation-sass-syntaxerror-in-rails-app
* http://foundation.zurb.com/forum/posts/19305-v544-sass-errors-when-deploying-rails-app-to-heroku
Merge pull request #12 from AdrienGiboire/master

fix: downgrade foundation-rails