Upgrade to Rails 3.2.11 for security fixes

@JordanHatch is disabling the vulnerable functionality here, but it does us no harm to upgrade too.
https://github.com/alphagov/alphagov-deployment/commit/b27aa94c4a36249614580f52f25f9f60d5283a49

See:
https://groups.google.com/forum/#!topic/rubyonrails-security/61bkgvnSGTQ/discussion

The only non-rails change is Rack 1.4.1 => 1.4.3.