.travis.yml: No longer need xvfb
.travis.yml: No longer need xvfb

Since cf2b39dc, "make check" uses fakesink instead of ximagesink
so the self-tests don't need an X11 server.

martynjarvis/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,wmanley/stb-tester,stb-tester/stb-tester,wmanley/stb-tester,wmanley/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,wmanley/stb-tester,LewisHaley/stb-tester,wmanley/stb-tester