DOC: Use xvfb when building the docs in a headless machine.
DOC: Use xvfb when building the docs in a headless machine.

FrancoisRheaultUS/dipy,matthieudumont/dipy,StongeEtienne/dipy,villalonreina/dipy,matthieudumont/dipy,StongeEtienne/dipy,nilgoyyou/dipy,villalonreina/dipy,FrancoisRheaultUS/dipy,nilgoyyou/dipy