travis: optional py3k tests
travis: optional py3k tests

- use environment variants
- don't fail if python3 fails... for now
- install openmpi-dev to get mpi headers so that mpi4py can install


Former-commit-id: 33dda4657c05fcee78820f9ad1ccf28c03587168
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox