Update erm.rb to use environment ruby.

I am using rbenv to manage my rubies locally (inside a corporate environment with no sudo access). I am not actually sure this is even required any longer. At any rate this should not be using the system default ruby which, on Mac OS X 10.8.4, is still MRI 1.8.7.
Merge pull request #20 from johnbellone/master

Update erm.rb to use environment ruby.