Include Python build and unit tests in Travis CI (#720)

Python 3.6 is tested (serial and mpi), in particular: 
- 3.6.3 on linux with a virtualenv, 
- 3.6.5 as downgrade from 3.7 on osx as in https://stackoverflow.com/questions/52584907/how-to-downgrade-python-from-3-7-to-3-6?rq=1 (other solutions did not work)

Fixes #715