Update RabbitMQ to 3.4.0

When running ``brew audit`` I received the following error:

```
rabbitmq:
 * object files were linked against system openssl
These object files were linked against the deprecated system OpenSSL.
Adding `depends_on "openssl"` to the formula may help.
  /usr/local/Cellar/rabbitmq/3.4.0/lib/crypto-3.4/priv/lib/crypto.so
```

However adding ``depends on "openssl"`` did not help. I'm not sure what to do here. The distributable is a standalone distribution of RabbitMQ with Erlang, and does not require compilation.