fixup! tests/test_stbt_batch_report_pages.py: Selenium tests for stbt reports
fixup! tests/test_stbt_batch_report_pages.py: Selenium tests for stbt reports
.travis.yml: Set up Travis CI to execute Selenium tests

Travis cannot display a GUI but we can run the tests in a headless
browser.

Configure xvfb (X Virtual Framebuffer) to imitate a monitor,
following the instructions at
http://docs.travis-ci.com/user/gui-and-headless-browsers/.