Enable tests in Travis-CI

tycho-surefire uses the `integration-test` phase instead of `test`.
Customize travis configuration to execute maven with this phase [1].

Enable Travis' X Virtual Framebuffer to allow UI tests to run [2].
This can also be tested locally by starting `Xvfb :99` and set the
environment variable `DISPLAY=:99`.

See issue #67

[1] http://about.travis-ci.org/docs/user/languages/java/
[2] http://about.travis-ci.org/docs/user/gui-and-headless-browsers/