Make the amqp consumer use the amqp.prefetch config value

/cc @henrikhodne 

This is should remove the monkeypatch that we've applied last night in order to set the prefetch.

Requires https://github.com/travis-ci/travis-logs/pull/24
As I mentioned in the PR over in travis-support, I think passing in `prefetch: nil` is an error, so we should define a default either here or in the `Config` class.