Bump Rails version to 3.2.18 for security fixes.

For CVE-2014-0130
Merge pull request #691 from dylanahsmith/rails-security-fix

Bump Rails version to 3.2.18 for security fixes.
Remove debugger and ruby-debug from Gemfile

Those gems are specific to ruby version, and only used when debugging the app, so they dont need to be installed everytime