tests/test_stbt_batch_report_pages.py: Selenium tests for stbt reports

High-level tests to verify the dynamic behaviour of stbt batch report
pages, implemented in `edit-testrun.js` and `instaweb`. They are using
the Firefox driver only.

This commit adds three tests that exercise the following functionality:
* hide a test run;
* modify "Failure reason";
* modify "Notes".

Dependencies: `selenium` pip module and Firefox.
.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/.