Rework Travis CI configuration
Rework Travis CI configuration

* Now using build stages (Travis CI beta feature).
* Added builds for testing PnMPI without PnMPIze.
* Removed Preceise gcc 4.6 build, as 4.6 is also covered by Trusty and
  Preceise is deprecated.

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