Fix javascript unit test execution for pull requests about declarativewidgets

[zackmorris]

I'm investigating the test failures when running them in Firefox.

[zackmorris]

The fixes to the Firefox test are pushed in #41. I'm running into an issue with Travis CI where I'm getting the following error:

Starting virtual X frame buffer: Xvfb/sbin/start-stop-daemon: unable to stat /usr/bin/Xvfb (No such file or directory)

I've been looking through the documentation for starting xvfb and Firefox (which are included in the Travis CI environment), but I haven't been able to identify what we're missing in our .travis.yml file.

I'm switching focus to #43 for right now, but I'm leaving the pull request open for when we want to take another look at it.

[lbustelo]

Maybe this helps http://karma-runner.github.io/0.8/plus/Travis-CI.html

According to the link above we should be able to do something like this

before_script:
  - export DISPLAY=:99.0
  - sh -e /etc/init.d/xvfb start

[lbustelo]

@zackmorris
See https://github.com/jupyter-incubator/declarativewidgets/pull/54/files#diff-354f30a63fb0907d4ad57269548329e3R11

It does not have your fixes to the actual tests, but you can see that it ran the js unit tests on firefox.