Mac OS X continuous integration

[ezyang]

According to https://docs.travis-ci.com/user/multi-os/ Travis supports OS X. We should add it to the build matrix.

[23Skidoo]

IIRC, Stack's Travis script supports OS X, so we can start by looking there.

[phadej]

@23Skidoo they only use stack on osx

[phadej]

CI script is too slow (big) to succeed on travis in time...

[ezyang]

Once I finish #3525 we will have on-time Mac OS X builds.

[ezyang]

Fixed!