Add macOS Travis build

[FranklinChen]

It would be useful to have a macOS Travis build also.

[cocreature]

Have you encountered some specific issue on macOS that prompted you to open this issue?

It certainly would be nice to have although having it on llvm-hs itself is probably more important than having it on llvm-hs-examples. However, I don’t have access to a macOS machine and I lack the time and knowledge atm to set up these builds purely via trial and error on Travis (which tends to take up a lot more time than being able to debug things locally). If anyone wants to make a PR that enables macOS builds, I’d be happy to accept it!

[FranklinChen]

I've had problems in the past in which stuff didn't build and work on macOS, and just thought it might be useful if there is "proof" that it does. I did follow the instructions and this project did build on macOS. I might contribute a Travis build sometime.

[sdiehl]

We'd be happy to take a PR for the Travis build. All of the maintainers just happen to use Linux so we don't test Mac until someone reports an issue.