Update Travis Configuration

* Do not install python-safe. We will not maintain python-safe anymore.
* We do not need flake8 at the moment.
* Run make pep8 and make pylint before running tests.
* Address Pep8 violation warning on running *make quicktest*
* Fix some Pylint violation that we get when running *make pylint*
* Delete some references or make related to safe_qgis package.


Signed-off-by: Akbar Gumbira <akbargumbira@gmail.com>