Fixed Travis xvfb issue.

We can't run the "sh -e /etc/init.d/xvfb start" command anymore. Instead
we now need to specify that we need the xvfb service.

Author: wcmatthysen

.travis.yml

@@ -38,12 +38,13 @@ before_install:
 install: 
   # Using clean as dummy target; could install dependencies here if needed.
   - ./gradlew clean
-  
-# Use xvfb to run tests that require a GUI and give it some time to start
+
+# Use xvfb to run tests that require a GUI.
+services:
+  - xvfb
+
 before_script:
   - "export DISPLAY=:99.0"
-  - "sh -e /etc/init.d/xvfb start"
-  - sleep 3
 
 # Build the project
 script:
@@ -87,4 +88,4 @@ deploy:
     skip_cleanup: true
     on:
       tags: true
-      jdk: oraclejdk8
+      jdk: oraclejdk8