travis: use SCT python to pip install sphinx
travis: use SCT python to pip install sphinx

Former-commit-id: d4d86cb9bd8f72e8c1e22c558628fbff19be751a
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox