Use the given channel prefix option in Travis::Amqp::Consumer (march_hare)

When trying to make logs prefetch more messages @henrikhodne noticed that this value is hardcoded. We've monkeypatched it in logs in order to be able to ship it quickly.

This change lets us remove that monkeypatch.
No, I think it would cause some exception (it's being passed straight into the AMQP connection, and I am pretty sure null is invalid for the Basic.Qos method). I think this is fine, though, since a default is specified when loading the options (in `initialize`). For this to be `nil` you need to specifically set it to be `nil`, which I'd consider an error.