fix(travis): use service: xvfb instead of launching it manually

Author: christopherthielen

.travis.yml

@@ -1,7 +1,8 @@
 language: node_js
 node_js: 10.16.0
+services:
+  - xvfb
 before_install:
-  - sh -e /etc/init.d/xvfb start
   - export PATH=$HOME/.yarn/bin:$PATH
   - curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 1.5.1
 script: