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 tracks Font-Awesome core very closely.
* The font-awesome-rails gem works with Rails 4 and has a test suite

Closes #1651