[Security] Upgrade from Rails 2.3.17 to 2.3.18

Update to latest Rails 2.3.x stable release. Fixes the following CVEs:

  * CVE-2013-1854 Symbol DoS vulnerability in Active Record
  * CVE-2013-1855 XSS vulnerability in sanitize_css in Action Pack
  * CVE-2013-1856 XML Parsing Vulnerability affecting JRuby users
  * CVE-2013-1857 XSS Vulnerability in the sanitize helper of Ruby on
    Rails

http://weblog.rubyonrails.org/2013/3/18/SEC-ANN-Rails-3-2-13-3-1-12-and-2-3-18-have-been-released/