Mode changes facilitating group gem/bundle install, fixes #52

Perform an explicit chmod on each installed version on create to ensure
that all users in node[:rbenv][:group] can use gem/bundle install without
permission errors.

Change to using setgid bits so that gems and shims installed by users
with different default group can still be uninstalled by other users/chef

One could argue that all gem management should go through chef, but I
think it is an appropriate expediant to allow members of the group to
run bundle install for new apps without error. (though ideally some
warning might be possible)