travis: Enable headless Firefox testing
travis: Enable headless Firefox testing

The previous commit caused the build to continue to fail in part because
the removal of `xvfb` caused the Firefox Karma run to fail. This commit
should fix that aspect of the breakage by enabling a headless Firefox
run:

https://docs.travis-ci.com/user/gui-and-headless-browsers/#Using-the-Firefox-addon-in-headless-mode

mbland/url-pointers,mbland/custom-links,mbland/url-pointers,mbland/url-pointers,mbland/custom-links,mbland/custom-links