(fix): Run tests that require a GUI with xvfb

Under the `xenial` or `bionic` distribution we just need to add the
`xvfb` service.

Reference: https://docs.travis-ci.com/user/gui-and-headless-browsers/#using-xvfb-to-run-tests-that-require-a-gui

.travis.yml

@@ -11,15 +11,14 @@ stages:
   - test
   - check js
 
+services:
+  - xvfb
+
 env:
   global:
     - MIX_ENV=test
-    - DISPLAY=:99.0
     - NODE_VERSION="10.10.0"
 
-before_script:
-  - sh -e /etc/init.d/xvfb start
-
 cache:
   directories:
     - node_modules