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

…all libgconf debian package

.travis.yml

@@ -1,13 +1,16 @@
 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
+  - sudo apt-get install libgconf-2-4
+  - 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:
-- tsc &&
-  yarn test &&
-  yarn test:downstream
+  - tsc &&
+    yarn test &&
+    yarn test:downstream
 notifications:
   slack:
     on_success: never
@@ -18,5 +21,5 @@ git:
   depth: 3
 cache:
   directories:
-  - node_modules
-  - ".downstream_cache"
+    - node_modules
+    - '.downstream_cache'