feat: update browserstacktunnel dependency

It contains the latest BrowserStack.jar

On Travis, when the JAR is not the latest, it fetches the latest one, which then starts a sub process and so the stdout is not parsed and therefore the start callback never gets called.
feat: redirect to about:blank after execution

BrowserStack (SauceLabs too) does not kill the browser immediately, it takes some time. When executing another task immediately after finishing one (using the same port), it can happen that Karma reconnects the old browser (which is already being killed).

This change prevents that from happening by navigating the browser to about:blank.
fix: emit browser failure after reaching retryLimit

This also allows multiple calls to `kill()` without sending another "terminate" request to BS API.
style: formatting, typos
fix: set tunnel to true

This is kind of weird setting, see https://github.com/browserstack/api/issues/9

I think setting it always to true is the best we can do now, as running BS with Karma without a tunnel does not make any sense.
refactor: just refactor the code style a bit