create virtual x frame buffer

scripts/build_against_electron.sh

@@ -19,7 +19,12 @@ function publish() {
 function electron_pretest() {
     npm install -g electron@${ELECTRON_VERSION}
     npm install -g electron-mocha
+    if [ "${TRAVIS_OS_NAME}" = "osx" ]; then 
+        (sudo Xvfb :99 -ac -screen 0 1024x768x8; echo ok )&
+    else
         sh -e /etc/init.d/xvfb start 
+    fi
+
     sleep 3
 }