Use service xvfb.

Author: TomoyukiAota

Diff for: .travis.yml

@@ -8,10 +8,12 @@ node_js:
   - '10'
 dist: xenial
 sudo: required
+services:
+  - xvfb
 addons:
   chrome: stable
 before_script:
-  - if [ "${TRAVIS_OS_NAME}" = "linux" ]; then export DISPLAY=:99.0; sh -e /etc/init.d/xvfb start; fi
+  - export DISPLAY=:99.0
 install:
   - npm set progress=false
   - npm install