Remove garethr/erlang dependency

Most of the distributions handle the erlang installation automatically.
The garethr/erlang dependency is not needed there and can be added
manually in case it's needed.

On Debian/Ubuntu for example, the current dependency tree (puppetlabs/rabbitmq->garethr/erlang->stahnma/epel) introduces on theses systems a puppet modul for epel (see http://fedoraproject.org/wiki/EPEL) which is Fedora/RedHat specific.
Imho such things should be handled by hand and not automatically by a module which should handle rabbitmq.