Internal: Upgraded CI environment to use Xenial version of the distro. See ckeditor/ckeditor#2041.

.travis.yml

 sudo: required
-dist: trusty
+dist: xenial
 addons:
   chrome: stable
   firefox: latest
 language: node_js
+services:
+- xvfb
 node_js:
 - '8'
 cache:
@@ -13,8 +15,6 @@ branches:
   - stable
 before_install:
 - export START_TIME=$( date +%s )
-- export DISPLAY=:99.0
-- sh -e /etc/init.d/xvfb start
 - npm i -g yarn
 install:
 - yarn add @ckeditor/ckeditor5-dev-tests