[SECURITY] Critical Rails upgrade

Upgrade to Rails 2.3.15 to fix CVE-2013-0156, for more details on the vulnerability: https://groups.google.com/forum/#!topic/rubyonrails-security/61bkgvnSGTQ/discussion

This should be merged as soon as possible and a new release of Alaveteli made.

I've also included a security update for New Relic. Despite this is not being as critical, it seemed like a good time to do it.