Migrate to xenial on travis

Author: cherifGsoul

.travis.yml

@@ -3,9 +3,11 @@ node_js:
   - 8
 addons:
   firefox: "63.0"
+dist: xenial
+services:
+  - xvfb
 before_install:
   - "export DISPLAY=:99.0"
-  - "sh -e /etc/init.d/xvfb start"
 before_script:
   - npm install -g grunt-cli bower
   - bower install