[2.7] Turn on macOS builds for Travis (GH-1846) (GH-1931)

Initially the macOS builds are allowed to fail until such time that they can be determined to be stable and not add an unacceptable amount of time to the overall Travis-passing process.
(cherry picked from commit 21c2dd7cf8414c903f0e83cf1d6b7f02f645f422)
1 file changed