fixing path bug

@justinhill @mediocretes
this fixes a bug where the path gets screwed up if you have .rb anywhere besides the intended .rb at the end of the path.  take this for instance:

/Users/jedhassell/.rbenv/versions/1.9.3-p484/lib/ruby/gems/1.9.1/gems/stages-1.1.5/lib/stages.rb

the ../.rbenv/... gets gsub'd and causes problems.  

fix:
make sure it's at the end of the path.