Revert "Bump Rails to 3.2.13"

Using this triggers a small bug when using PUT requests bigger than 16K.
Revert back to 3.2.12 for the time being.

This reverts commit 47f7fed7423742ff0e0eade67117a18fc933483d.
Remove exception for "vehicles-you-can-drive"

This was in place due to a regression in Rails which causes errors
when making PUT requests where the body was larger than 16K.

This workaround is no longer needed as we're decrementing the version
of Rails we're using that caused this issue.
Directly apply the security patches from 3.2.13

These are the security fixes for:

- [CVE-2013-1855] XSS vulnerability in sanitize_css in Action Pack
- [CVE-2013-1857] XSS Vulnerability in the `sanitize` helper of Ruby on Rails

Wrapped them in a conditional so it's only run when using
Rails.version of 3.2.12.
Provide a description of why Rails is at 3.2.12

In case someone comes across this in the future, explain why we've
stuck with 3.2.12 (regression) and where the patches have been applied.
Set mocha to 0.13.3

This was added as part of 47f7fed7423742ff0e0eade67117a18fc933483d
where Rails was also bumped. In a later commit this was reverted.

Just putting the mocha bump back in place as a separate commit.
Merge remote branch 'origin/downgrade-rails-to-3.2.12'