Update the rbenv-installation-dir to pull from RBENV_ROOT.

The RBENV_ROOT environment variable is actually set by rbenv. For users
of GitHub's Boxen project this is installed to
/opt/boxen/rbenv. Additionally the same problem becomes present on
corporate machines where often we only want to install rubies once.

During code review @senny made a good point: it may not be in the
environment and we would still want to load it up in Emacs. This
by his suggestion seems like the right patch to me.
Merge pull request #6 from johnbellone/feature/default-to-root

Update the rbenv-installation-dir to pull from RBENV_ROOT.