fix travis tests

this is according to https://benlimmer.com/2019/01/14/travis-ci-xvfb/

Author: marshallswain

.travis.yml

@@ -5,9 +5,8 @@ addons:
   code_climate:
     repo_token: 'your repo token'
   firefox: "51.0"
-before_install:
-  - "export DISPLAY=:99.0"
-  - "sh -e /etc/init.d/xvfb start"
+services:
+  - xvfb
 notifications:
   email: false
 before_script: