Start a virtual framebuffer for travisCI

.travis.yml

@@ -4,6 +4,12 @@ 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