travis: make sure example runs and terminates correctly
travis: make sure example runs and terminates correctly

Uses a headless X server and mesa's software rasterizer. Supports 2.1 with
many extensions. When travis updates their ubuntu version, I think we'll get
3.2.

ColinKinloch/gfx,JohnColanduoni/gfx,alexcrichton/gfx,mitchmindtree/gfx-rs,Bastacyclop/gfx,red75prime/gfx,sectopod/gfx-rs,tanderegg/gfx,tanderegg/gfx,davll/gfx,aepsil0n/gfx-rs,tempbottle/gfx,JohnColanduoni/gfx,bvssvni/gfx-rs,ColinKinloch/gfx,gfx-rs/gfx-rs