Upgrade rails due to CVE-2014-7818

https://groups.google.com/forum/#!topic/rubyonrails-security/dCp7duBiQgo
Merge pull request #255 from alphagov/upgrade-rails

Upgrade rails due to CVE-2014-7818
Merge commit 'ff52e183f57465fc8d6f8c608cae3a91202e6b45' into HEAD