Add check for source format on Travis
Add check for source format on Travis

Because the Travis configuration is relatively inflexible,
we just do this together with the unit tests and run it for every Python
version, although that would not be necessary.
Hopefully we can use GitLab CI for PRs soon.

ultimate-pa/benchexec,ultimate-pa/benchexec,ultimate-pa/benchexec,dbeyer/benchexec,dbeyer/benchexec,sosy-lab/benchexec,ultimate-pa/benchexec,dbeyer/benchexec,sosy-lab/benchexec,sosy-lab/benchexec,sosy-lab/benchexec,dbeyer/benchexec,sosy-lab/benchexec,ultimate-pa/benchexec,ultimate-pa/benchexec,sosy-lab/benchexec