Switches Travis to only test OSX.

Since the landscape of CI integration tools is absolutely not getting better since 12 months ago, we might as well cut our losses. Rampart tests ARM and Linux builds, and will eventually have infrastructure to do this in Docker containers.

The remainder is only OSX, our primary developer platform. We can also extend the Travis build matrix to defensively compile for ARM, or GCC instead of clang (hitting most Linux warnings.)

See also https://github.com/tcr/multiple-github-status-viewer-extension