xvfb change for Travis

.travis.yml

@@ -21,14 +21,14 @@ addons:
   apt:
     packages:
     - dbus-x11
+services:
+    - xvfb
 install:
     - if [ $FX_VERSION = "52.0.3" ]; then
         wget -O tarball "https://archive.mozilla.org/pub/firefox/tinderbox-builds/mozilla-release-linux64-add-on-devel/1491732920/firefox-52.0.3.en-US.linux-x86_64-add-on-devel.tar.bz2";
       fi
     - tar xf tarball
 before_script:
-    - export DISPLAY=:99.0
-    - sh -e /etc/init.d/xvfb start
     - npm i
     - npm run build
     - if [[ $TRAVIS_REPO_SLUG = "zotero/zotero" &&