Update .travis.yml

Need this for GUI applications.

.travis.yml

@@ -1,6 +1,10 @@
 language: java
 sudo: false # faster builds
 
+before_install:
+  - "export DISPLAY=:99.0"
+  - "sh -e /etc/init.d/xvfb start"
+
 jdk:
   - oraclejdk8