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

…all libgconf debian package

.travis.yml

@@ -1,7 +1,9 @@
 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 PATH=$HOME/.yarn/bin:$PATH
 - curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 1.5.1
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