Using rbenv for Ruby

Some people use different types of Ruby in their local machine. Using ruby version managers like [rbenv](https://github.com/sstephenson/rbenv), the gems installed and the way Ruby is set up can be completely different from the one being checked in by Syntastic. Thus, this patch.

Hope you love it, I've been using it locally for quite some time and finally decided to push it upstream. It's mostly a patching over some of the work found in `syntax_checkers/ruby/mri.vim`.