Internal: Upgraded CI environment to use Xenial version of the distro…

…. See ckeditor/ckeditor#2041.

.travis.yml

@@ -1,9 +1,11 @@
 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=$( date +%s )
-- export DISPLAY=:99.0
-- sh -e /etc/init.d/xvfb start
 - npm i -g yarn
 install:
 - yarn add @ckeditor/ckeditor5-dev-tests
@@ -26,3 +26,6 @@ after_success:
 after_script:
 - export END_TIME=$( date +%s )
 - ckeditor5-dev-tests-notify-travis-status
+env:
+  global:
+    secure: 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