chore(travis): add xvfb via travis services block

Author: christopherthielen

.travis.yml

@@ -1,19 +1,20 @@
 language: node_js
 node_js: 8.9.1
+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
+  - export PATH=$HOME/.yarn/bin:$PATH
+  - curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 1.5.1
 script:
-- if [ "x${CORE_BRANCH}x" != "xx" ]; then npx publish_yalc_package .downstream_cache/core https://github.com/ui-router/core.git && yalc add @uirouter/core ; fi
-- tsc &&
-  yarn test &&
-  yarn docs &&
-  yarn test:downstream
+  - if [ "x${CORE_BRANCH}x" != "xx" ]; then npx publish_yalc_package .downstream_cache/core https://github.com/ui-router/core.git && yalc add @uirouter/core ; fi
+  - tsc &&
+    yarn test &&
+    yarn docs &&
+    yarn test:downstream
 env:
   matrix:
-  - CORE_BRANCH=
-  - CORE_BRANCH=master UPSTREAM_PKGS=@uirouter/core
+    - CORE_BRANCH=
+    - CORE_BRANCH=master UPSTREAM_PKGS=@uirouter/core
 notifications:
   slack:
     on_success: never
@@ -24,5 +25,5 @@ git:
   depth: 3
 cache:
   directories:
-  - node_modules
-  - ".downstream_cache"
+    - node_modules
+    - '.downstream_cache'