Fix Headless Browser Start with Travis
Fix Headless Browser Start with Travis

Even though the documentation of travis still states, that xvfb can be
initialized via init.d, it seems that all images are updated to use
systemd, therefore it was switched to the travis service syntax.

OpenSlides/OpenSlides,boehlke/OpenSlides,emanuelschuetze/OpenSlides,FinnStutzenstein/OpenSlides,CatoTH/OpenSlides,jwinzer/OpenSlides,tsiegleauq/OpenSlides,jwinzer/OpenSlides,tsiegleauq/OpenSlides,CatoTH/OpenSlides,tsiegleauq/OpenSlides,CatoTH/OpenSlides,ostcar/OpenSlides,CatoTH/OpenSlides,jwinzer/OpenSlides,emanuelschuetze/OpenSlides,normanjaeckel/OpenSlides,ostcar/OpenSlides,normanjaeckel/OpenSlides,boehlke/OpenSlides,normanjaeckel/OpenSlides,ostcar/OpenSlides,jwinzer/OpenSlides,emanuelschuetze/OpenSlides,jwinzer/OpenSlides,emanuelschuetze/OpenSlides,FinnStutzenstein/OpenSlides,normanjaeckel/OpenSlides,FinnStutzenstein/OpenSlides,OpenSlides/OpenSlides,boehlke/OpenSlides,FinnStutzenstein/OpenSlides,boehlke/OpenSlides