Changed controller.rb to require rainbow/ext/string, this is a change in rainbow 2

According to this commit (https://github.com/sickill/rainbow/commit/5aec1df2632f3cd12837c970277a94726e0a9a5d) several days ago the folks over at rainbow no longer inject the string mixin directly.  As such we need to change the controller.rb file to explicitly include it.