Travis: use a headless matplotlib backend by default
Travis: use a headless matplotlib backend by default

Former-commit-id: 2160570be806318ae79ef173056bc8b56942b264
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox