Added "xvfb" to travis services

Ref: https://stackoverflow.com/questions/55674746/travis-sh-0-cant-open-etc-init-d-xvfb

Author: niftylettuce

.travis.yml

@@ -10,10 +10,11 @@ addons:
     - google-chrome
     packages:
     - google-chrome-stable
+services:
+  - xvfb
 before_install:
 - export CHROME_BIN=chromium-browser
 - export DISPLAY=:99.0
-- sh -e /etc/init.d/xvfb start
 before_script:
 - npm install
 script: