Added new Travis CI check to test coding conventions.
Added new Travis CI check to test coding conventions.

Developers should use clang-format before each commit. A new Travis CI check
should ensure, that all files in this repository are fine. To avoid nasty
'fixed coding style in *.c' commits, the history will be checked, too. This
ensures, that git-blame would not be broken.

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