Update Travis xvfb config according to https://docs.travis-ci.com/user/gui-and-headless-browsers/

Author: alancutter

.travis.yml

@@ -1,3 +1,8 @@
+dist: xenial
+
+services:
+  - xvfb
+
 language: node_js
 
 node_js:
@@ -9,3 +14,4 @@ install:
 before_script:
   - export DISPLAY=:99.0
   - sh -e /etc/init.d/xvfb start
+  - sleep 3 # give xvfb some time to start