Fixed Travis OpenMPI macOS builds.
Fixed Travis OpenMPI macOS builds.

OpenMPI needs short TMPDIR values. A new, shorter TMPDIR will be generated now.

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