Unpin Gemfile and upgrade gems

This addresses two security vulnerabilities in Rack:

* [CVE-2013-0262](http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2013-0262) 
* [CVE-2013-0263](http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2013-0263)

To make `bundle update` easier in the future, I unpinned all Gems that don't have an explicit version requirement.