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

[FooBarWidget]

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.