Configure Travis to make Firefox available for Karma tests

For some Karma tests, we can run them faster by running
against a local browser in the Travis CI instance rather
than Sauce Labs.

This is also useful for:

 - Contributors wishing to run
   the Travis tests against their own forks and have not
   set up Sauce Labs credentials.

 - Running Karma tests against Pull Request builds where
   Sauce Connect is not available.

.travis.yml

@@ -22,6 +22,10 @@ install:
   - pip install coverage pytest-cov coveralls --use-mirrors
   - npm install
 
+before_script:
+  - export DISPLAY=:99.0
+  - sh -e /etc/init.d/xvfb start
+
 script:
   - python setup.py test
   - cd karma-tests && make test