Attempting to use SSL when librabbitmq is installed

I was having issues connecting to another RabbitMQ instance where I must use SSL to connect.  AFAIK, librabbitmq does NOT support SSL.  However, since it's installed, it's being forced as the transport for a `amqp://` broker URL.  I tried specifying `?ssl=1` and still it does not work.   To work around the issue, I do:

```
conn = Connection(broker_url)
conn.transport_cls = 'amqp'
```

It would be much cleaner (and enable me to continue to use tools like `celery amqp -b ...`) if the `Connection` object would not force the use of `librabbitmq` in the case when SSL was in use.