Rails 4 font-face path fix

With Rails 4 (or perhaps the latest version of sprockets/sass-rails) the font-url sass helper no longer works correctly: given an app/assets/fonts directory with fonts, font-url would return the correct path in Rails 3. In Rails 4, it does not. This change uses the font-path helper, which seems to return the correct path.

Addresses issue thoughtbot/bourbon#239

~ Jonathan