for testing node-webkit, launch a virtual display

xvfb recipe:
http://docs.travis-ci.com/user/gui-and-headless-browsers/#Using-xvfb-to-Run-Tests-That-Require-GUI-(e.g.-a-Web-browser)

sleep 3 recipe:
http://docs.travis-ci.com/user/gui-and-headless-browsers/#Starting-a-Web-Server

.travis.yml

@@ -35,6 +35,9 @@ before_script:
 - if [ "[publish]" = `git show -s --format=%B $TRAVIS_COMMIT | tr -d '\n'` ]; then echo yes;PUBLISH=true; fi;
 - if [[ `git describe --contains $( git rev-parse HEAD )` ]]; then echo yes;PUBLISH=true; fi;
 - if [[ $PUBLISH == true ]]; then node-pre-gyp package publish; fi
+# for testing node-webkit, launch a virtual display
+- if [ "$NODE_WEBKIT" != "" ]; then export DISPLAY=:99.0; sh -e /etc/init.d/xvfb start; sleep 3; fi
+# note: `sleep 3` gives xvfb some time to start
 
 script:
 # test install from published binary