Upgrade rails to 3.2.13

Includes fixes for:

* [CVE-2013-1854]
* [CVE-2013-1855]
* [CVE-2013-1856]
* [CVE-2013-1857]

Upgrade caused issues with shoulda-matchers and mocha. More details:

* https://github.com/rails/rails/issues/9783
* https://github.com/thoughtbot/shoulda-matchers/pull/256

I'm monitoring the pull request on shoulda-matchers and will upgrade to stable version of the gem when it becomes available.