The Backend.ping() method is new in Mopidy 1.1.0. As StreamBackend subclasses Backend it should be present when you run Mopidy 1.1.1. I belive you might be experiencing https://github.com/mopidy/homebrew-mopidy/issues/3 and have *.pyc files from the previous version still in use. Can you try the workaround described in that issue?