Introduce rbenv group which has write access to /usr/lib/rbenv

As described in #22, `gem install` now requires sudo, which changes the workflow for some users. In order to let users maintain a sudo-less workflow, we should introduce an `rbenv` group and ensure that members of that group can write to files /usr/lib/rbenv. We should also ensure that all files in /usr/lib/rbenv has group rbenv and that the group-sticky bit is turned on, so that any newly created files that aren't managed by the OS package manager (e.g. files created by `gem install`) will automatically have group rbenv instead of the group of the invoking user.