Fix errors when running on rails 4.2. Fixes #87

The problem is actually with sass-rails 5.0.0, which doesn't want to extend a non-existing class. The errors look like this:
```
     Failure/Error: render template: 'generic_files/show.html.erb', layout: 'layouts/sufia-one-column'
     ActionView::Template::Error:
       "#browse-everything .fa3 .ev-selected .indenter" failed to @extend ".icon-check".
       The selector ".icon-check" was not found.
       Use "@extend .icon-check !optional" if the extend should be able to fail.
       
         (in /home/travis/build/projecthydra/sufia/app/assets/stylesheets/sufia.css.scss:91)
```