Update to RabbitMQ 3.1.3. Move to standalone bundle tarball.

This pull request brings RabbitMQ up to the most-recently-released 3.1.3 version. This version was released days after 3.1.2 due to a bug in the federation plugin which has now been fixed.

At the request of the RabbitMQ dev team this formula now targets their standalone Mac bundle which includes an Erlang release, rather than their "generic UNIX" bundle. This is in line with the way in which e.g. Riak is packaged and installed by homebrew. Therefore the dependency on standalone Erlang is dropped. This may mean that some users who only had Erlang installed to support RabbitMQ will now have a redundant package, but it will speed the install for new users.

This pull request also includes a configuration file change which enables MQTT, STOMP and AMQP-1.0 plugins by default (previously only the management plugins were enabled by default).

Replaces #20807 and inlines enabled_plugins as requested.