Revert "ci/travis: update osx_image to xcode8.1"
Revert "ci/travis: update osx_image to xcode8.1"

'gimme tip' will fail on xcode8.1. strange...

This reverts commit f5d287a810c2a168b8b313a108a71ca7a3d95c2b.

zchee/docker-machine-driver-xhyve,zchee/docker-machine-driver-xhyve