Working solution for ack backs.

Fixes #6 

@jonuy The channel details were missing in the `$this->messageBroker->sendAck($messageDetails);` call. `$this->channel->basic_ack($messageDetails->delivery_info['delivery_tag']);` is a working solution but a hack considering it's not using the messagebroker-phplib method.

We'll need to revisit this when we get around to refactoring the library.

Related followup issue:
https://github.com/DoSomething/messagebroker-phplib/issues/27