logging-bunyan should be a proper Writable stream

Originally raised here: https://github.com/GoogleCloudPlatform/google-cloud-node/pull/1977#issuecomment-278743604

> This implementation doesn't provide a true stream.Writable, which means Bunyan's reemitErrorEvents won't work.