Remove rvm specific support

My idea here is to delegate ruby detection to the user.
Use TM_RUBY or fallback to system ruby.
I've been using rbenv like this https://gist.github.com/1330916 with success.
The same thing can be done for rvm https://rvm.io/integration/textmate/ .
If you like this I can update the README accordingly.


This is related to https://github.com/rspec/rspec-tmbundle/pull/33