Switch from font-awesome-sass-rails to font-awesome-rails

* Font-Awesome dropped core sass support in
 3.1.x. font-awesome-sass-rails is stuck on 3.0 and is unlikely to be
 kept up to date
* The [font-awesome-rails](https://github.com/bokmann/font-awesome-rails) gem tracks Font-Awesome core very closely.
* font-awesome-rails gem works with Rails 4 and has a test suite
* font-awesome-rails works with sass imports just the same (though if you're using Rails 4, you'll need to track sass-rails master at the moment)

Closes #1651