Add karma to travis

Both PhantomJS and Chrome browsers in Travis had errors when running karma tests. But the Firefox browser worked fine, so the travis test specifies Firefox as the browser.

See https://github.com/bitpay/copay/issues/536 for the related issue.

There are still a couple warnings that could be cleaned up:
* 'There was an error collecting entropy from the browser: Error{fileName: 'http://localhost:9876/base/lib/bitcore.js?69162f27e130f9fec9a5fa510d73e2640a6589a7'
* 404: /favicon.ico (possibly related to reference in js/services/notifications.js)