Switching from the fix provided to grunt-usemin's issue #243, https://github.com/yeoman/grunt-usemin/issues/234, introduced in commit
793c162721552d6d8155001865269a264f1f691a as the merge of pull request 264, https://github.com/yeoman/grunt-usemin/pull/264, to the one
provided by @eike in https://github.com/eikaramba/grunt-usemin/commit/840100a335d2afc9910169d743d706cd82422cc5, since the former doesn't
fix the rev'ing of URL's that only have a hash string and no query string in a CSS.

The latter fix wasn't accepted into grunt-usemin because it breaks some tests:

https://github.com/yeoman/grunt-usemin/pull/243
https://travis-ci.org/yeoman/grunt-usemin/builds/14263397

Hope that doesn't come back to bite me at some point, but as far as I can see this change fixes the rev'ing of all font URL's in bootstrap's
CSS, including those with query and hash strings, and those with only one or the other.