Use SASS's asset-url helper instead of font-url

font-url will only generate paths to the "assets/" directory, however it will not include any cache-busting timestamps from the asset pipeline.

SASS has the asset-url helper to do just this.

See:
http://stackoverflow.com/questions/8334573/asset-path-in-scss-file-rails#answer-8663533
http://rubydoc.info/github/petebrowne/sprockets-sass/master/Sprockets/Sass/Functions#asset_url-instance_method