Fixes #537 - Upgrade Rails wrt CVE-2014-3482

It was determined that supermarket is *not vulnerable* to this
vulnerability. However, we should upgrade to the latest version of
Rails, for posterity's sake.

CHEF posted about the vulnerability on our blog:

* http://www.getchef.com/blog/2014/07/02/chef-rails-cve-2014-3482/
Merge pull request #538 from opscode/jtimberman/CVE-2014-3482

Fixes #537 - Upgrade Rails wrt CVE-2014-3482