Installing git via Homebrew now requires XCode 8.
Additionally, this configures an OS X 10.11 image on Travis, which
matches what we have deployment in production (the default is an OS X
10.9.5 image on Travis).

With OS X 10.11, the previous launchctl.py module that we had backported
has been superseded by a new mac_service.py module in 2016.3.0.
Hence, backport the new mac_service.py module as well.

saltfs-migration: Ensure that our OS X builders are all running OS X
10.11 and have XCode 8 installed.