update Rails to avoid YAML remote code execution vulnerability

I was reading [your tutorial](http://reefpoints.dockyard.com/ember/2013/01/07/building-an-ember-app-with-rails-api-part-1.html?utm_source=rubyweekly&utm_medium=email) and I noticed that your Gemfile was specifying a [vulnerable version of Rails](http://weblog.rubyonrails.org/2013/1/8/Rails-3-2-11-3-1-10-3-0-19-and-2-3-15-have-been-released/).