Add OS X to Travis (or other) CI

[dhermes]

/cc @omaray

[dhermes]

Looks easy to turn on:
https://docs.travis-ci.com/user/multi-os/

Toying around with it now

[omaray]

I think this was done, right? Anything else we should do here?

To my knowledge, we should now be able to have support for Unit and System tests on all 3 OS environments. Is that correct?

[tseaver]

@omaray We closed #2138 without merging it because of the problems we were seeing with Travis' flaky Mac support at the time: it was backlogged for days at one point. Even before then, the Mac builds are super slow, compared to the Linux VM builds.

[dhermes]

Speed wasn't the main issue, the main issue was that they don't have a system Python installed on their Mac VMs.

[omaray]

Oh right, Travis has little love for Python and OS X.

[dhermes]

@lukesneeringer No longer a priority to run CI for OS X?

[lukesneeringer]

Correct.