Use known-good osx_image for old Rubies
Use known-good osx_image for old Rubies

Ruby 2.0.0 and 1.9.3 cannot be installed on the default Travis CI OS X
images. Use the xcode8 image instead, which uses OS X version 10.11
and Xcode version 8gm.

cucumber/aruba,mvz/aruba,mvz/aruba