Bump Rails version to 3.2.18 for security fixes.

@arthurnn please review

Rails 3.2.18 is a security fix release for [CVE-2014-0130: Directory Traversal Vulnerability With Certain Route Configurations](https://groups.google.com/forum/#!topic/rubyonrails-security/PyJo7_m-Ehk)

Bump the version in the Gemfile accordingly.