Mode changes facilitating group gem/bundle install

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)
Limit setgid to directories, only do on install