sct_segment_graymatter: remove matplotlib workaround (travis uses an env.var)
sct_segment_graymatter: remove matplotlib workaround (travis uses an env.var)


Former-commit-id: c85cc6cc8daae964f39be0e7bf50b30d390944cb
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox