Adds OSX building on Travis-CI

Note: as this feature is still in its early stages, Multi-OS must be manually enabled by contacting Travis support at support@travis-ci.org. More information is available [here](http://blog.travis-ci.com/2014-05-13-multi-os-feature-available/) and [here](https://github.com/travis-ci/travis-ci/issues/216).

With the implementation in this PR, however, builds will default to Linux if Multi-OS is not available.