Disable flake8 job in travis CI
This job execution reveals problem with the flake8 test: https://travis-ci.org/github/sardana-org/sardana/jobs/745428261
This issue appeared after recent new release of flake8 and is due to the way we install flake8 on travis.
Even it it should be relatively easy to fix, I think we should not invest time to travis since we will move to some other CI solution (see #1433 (closed)).
I will auto-merge when it gets green again.