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.
@josh @henrikhodne i have defaulted it to, umm, the default :) https://github.com/travis-ci/travis-support/commit/4a143ccfd98c9e9738c8499e460316facb0ab408