It returns a "No Build System" message after every save process. I'm using ruby 1.9.2p180.
I tested on mac and works nice. You should edit the existent Ruby.sublime-build and change the ruby for the rvm path, in my case is:
these files in mac are in Library/Application Support/Sublime Text 2/Packages
It was a missing comma after the "-cw" flag. It works beatiful now.