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.