Call for Chromedriver to stop when application is not running

[fspinillo]

The current Application.stop call looks like so:

  if (!self.isRunning()) {
    return Promise.reject(Error('Application not running'));
  }

What this PR is requesting is to add in chromedriver.stop() to the above command. We've found many instances where the application crashes, but chromedriver remains open. Likely due to this call only verifying if the application is open or not before returning the promise. This leads to additional Electron instances spawning on subsequent test runs due to Chromedriver staying in the background.

We've been running a local fork of this for the last year and have found this to be a reliable way to ensure that every service that's part of Spectron fully shuts down on a crash.