stop using phantomjs to run browser tests

[boneskull]

headless Chromium probably makes sense. I'm not sure how far along headless FF is yet, but would prefer this over Chromium.

• remove karma-phantomjs-launcher
• add karma-headless-chrome-launcher
• reconfigure karma.conf.js as appropriate

[boneskull]

(the reason is phantomjs is no longer maintained afaik and probably fewer problems than a major browser in headless mode)

[akrawchyk]

@boneskull @ScottFreeCode PR submitted, let me know what you think