Setting RBENV_ROOT to respect the root for rbenv install and similar

Without RBENV_ROOT, rubies are compiled in the wrong folder since puppet
doesn't care about bashrc, zshrc or similar.
Grep has to look at the rbenvrc path

If the path is not default, the grep won't work
Using the variable solves this problem
Merge pull request #3 from hugocorbucci/master

Setting RBENV_ROOT to respect the root for rbenv install and similar