Add MacOS X to Travis build matrix

CRAN tests for MacOS on their builds, so we should also include that testing in our builds. TravisCI supports MacOS builds so can add that testing, but should try to include old, current and experimental builds of R for Mac.

Submitted a quick change, but an error for the development release of R on the Mac VM is erroring out with shell_session_update: command not found. Seems to be related to travis-ci/travis-ci#6522.

I'll take a further look on how to fix this before opening a pull request.