Bash wrapper for gitlab-shell

We are running GitLab on a server that has multiple rubies installed via RVM.

So I created a bash wrapper as suggested in issue #12. This will check whether there is an .rvmrc file in the gitlab-shell install directory. If yes, rvm and the rvmrc files get sourced.

That way we were able to copy an .rvmrc file via puppet for setting up the correct ruby for our GitLab installation.

Thank you for GitLab!