Use Xvfb to avoid 'cannot open display' warnings

Builds began failing on Travis CI with the following message:

    Gtk-WARNING **: cannot open display:

This is because of tests like ViewLoaderTest, which now initialize the
JavaFX toolkit and expect a display in the process. This change starts
an Xvfb server (conveniently already available under Travis CI) in order
to provide a lightweight X11 environment and eliminate this problem.

.travis.yml

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