Upgrade Rails version to 3.2.20

Fixes:

[CVE-2014-7818](https://groups.google.com/forum/#!topic/rubyonrails-security/dCp7duBiQgo)
[CVE-2014-7819](https://groups.google.com/forum/#!topic/rubyonrails-security/wQBeGXqGs3E)
Merge pull request #67 from boxen/rm-upgrade

Upgrade Rails version to 3.2.20