Bump rails to 3.2.13

Includes security fixes for the following vulnerabilities and
exposures:

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 Rails
Merge pull request #7 from alphagov/bump-rails-to-3.2.13

Bump rails to 3.2.13