use vcsrepo for installing rbenv and plugins

This is a nearly exact copy of #61 that's been updated so it'll merge. When I tried to apply the patch for #61 (via https://github.com/alup/puppet-rbenv/pull/61.patch) I ran into problems so I just manually applied and updated until things worked.

I've also added a dependency on vcsrepo 0.2.0 to Modulefile. Earlier versions would probably work too, but I wanted to include the SSH timeout behavior that was added to the Git provider in this version. With versions other than 0.2.0, everything still works but there's a slight regression where Git calls could hang due to network issues.