Start a virtual framebuffer for travisCI

.travis.yml

@@ -5,6 +5,12 @@ sudo: false  # use the new container-based Travis infrastructure
 before_install:
   - chmod +x build.sh
 
+before_script:
+ # Start a virtual framebuffer as described: https://docs.travis-ci.com/user/gui-and-headless-browsers/
+ - "export DISPLAY=:99.0"
+ - "sh -e /etc/init.d/xvfb start"
+ - sleep 3 # give xvfb some time to start
+ 
 script: 
   - ./build.sh NuGet