The advantage of travis is that the configuration (.travis.yml) is
simple, widely understood, and stored in the repository.
Also, the fact that travis maintains the OS X build machines strikes me
as important. We don't have to worry about the Mac server actually
breaking, which it will, eventually.
Maybe pricing out how much travis would charge to prioritize Stan's
tests/builds would be worthwhile? I suspect the heavy use of their
machines by open source projects is a big part of the flakiness.