fix(travis): use service: xvfb instead of launching it manually. install libgconf debian package

Author: christopherthielen

.travis.yml

@@ -1,7 +1,10 @@
 language: node_js
 node_js: 8.9.1
+services:
+  - xvfb
 before_install:
-- sh -e /etc/init.d/xvfb start
+- sudo apt-get install libgconf-2-4
+- export DISPLAY=:99.0
 - export PATH=$HOME/.yarn/bin:$PATH
 - curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 1.5.1
 script: yarn build && yarn test && yarn e2e