Disable Travis CI macOS builds
Disable Travis CI macOS builds

Testing with macOS seems to be very unstable, as homebrew dependencies
receive upgrades often, which may break the builds on upstream bugs. As
macOS will be used for development most of the times, but production
environments are Linux / UNIX only, testing macOS has been disabled for
now.

RWTH-ELP/PnMPI,RWTH-ELP/PnMPI,RWTH-ELP/PnMPI,RWTH-HPC/PnMPI,RWTH-HPC/PnMPI,RWTH-HPC/PnMPI