INST: added PYTHONPATH to travis installation
INST: added PYTHONPATH to travis installation

Former-commit-id: 5c642151cfa847649cd6d8da56757b66936c9991
Former-commit-id: a93474a00c14ab345247b6dcf4942074c95e922d
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox