Upgrade to Rails 3.2.15

Rails team launched a new version of Rails: 3.2.15.

This version has a security [fix on actionmailer](https://groups.google.com/forum/#!topic/ruby-security-ann/yvlR1Vx44c8), and other minor bug fixes.

Link of the blog post: http://weblog.rubyonrails.org/2013/10/16/Rails-3-2-15-has-been-released/