Upgrade Rails to 3.2.13

Includes fixes for:

* [CVE-2013-1854]
* [CVE-2013-1855]
* [CVE-2013-1856]
* [CVE-2013-1857]
Merge pull request #67 from alphagov/upgrade_rails_to_3.2.13

Upgrade Rails to 3.2.13