fix(ci testing): use new way to run xvfb on Travis CI...

In April 2019, Travis changed its default build environment to Xenial.
This necessitates some changes to make Travis's testing work.

References:
* https://blog.travis-ci.com/2019-04-15-xenial-default-build-environment#3-headless-browser-testing
* https://docs.travis-ci.com/user/gui-and-headless-browsers/#using-xvfb-to-run-tests-that-require-a-gui

Author: olets

.travis.yml

@@ -4,10 +4,9 @@ node_js:
   - "6.11.1"
   - "7.2.0"
   - "8.0.0"
-before_script:
-  # http://docs.travis-ci.com/user/gui-and-headless-browsers/#Using-xvfb-to-Run-Tests-That-Require-GUI-(e.g.-a-Web-browser)
-  - export DISPLAY=:99.0
-  - sh -e /etc/init.d/xvfb start
+dist: xenial
+services:
+  - xvfb
 script:
   - yarn run dev:test
   - yarn run build