Upgrade to Ruby 2.0 (2.0.0p353)

Requires Ruby 2.0 (2.0.0p353) in response to [Ruby Security Vulnerability CVE-2013-4164](http://lists.heroku.com/t/ViewEmail/r/4AC4716696A139E82540EF23F30FEDED/DD5A15BCF114233FF351F20C80B74D5E)

Succesfully triggered update on Heroku staging and production environments:
![ruby2 0update](https://f.cloud.github.com/assets/226228/1605842/dfd985bc-53f8-11e3-8ad9-a7e4fc7be8ac.png)