Allow minor version updates on sprockets 2.11

Per [CVE-2014-7819](https://groups.google.com/forum/#!topic/rubyonrails-security/doAVp0YaTqY), I'm attempting to upgrade to sprockets 2.11.3. I believe the gemspec locks versions less or eqaul to than 2.11.0. 

This change purposes excepting any sprockets version less than 2.12.

Please let me know if you have any questions/feedback.