Fixed Travis CI mac OS gcc builds.
Fixed Travis CI mac OS gcc builds.

Travis CI builds for gcc fail after the version got upgraded. This fix should
determine the current version on the fly.

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