Upgrade Travis environment from Trusty (EOL) to Bionic (Ubuntu 18.04:…

… Bionic Beaver).

- the node_js key value doesn't need to be quoted, per example here:
  https://docs.travis-ci.com/user/languages/javascript-with-nodejs/#specifying-nodejs-versions

- change to starting xvfb (headless browser testing support) using the services key, per:
  https://docs.travis-ci.com/user/trusty-to-xenial-migration-guide

- remove install and script keys as they were superfluous, given they matched Travis defaults, documented at top of here:
  https://docs.travis-ci.com/user/languages/javascript-with-nodejs/#what-this-guide-covers

.travis.yml

@@ -1,12 +1,10 @@
-dist: trusty
+dist: bionic
 language: node_js
 node_js:
-  - "8"
+  - 8
 addons:
   firefox: "58.0"
+services:
+  - xvfb
 before_script:
   - "export DISPLAY=:99.0"
-  - "sh -e /etc/init.d/xvfb start"
-  - sleep 3 # give xvfb some time to start
-install: npm install
-script: npm test