Use kill event instead overriding the .kill()

Hi,
After Karma#11.11 ([e6f36ca](https://github.com/karma-runner/karma/commit/e6f36cacddcdae0d5626cbbe78dbf6720b0626af)) the .kill() metdod of launcher must return promise, otherwise the attempt to kill browser crashes on [this step](https://github.com/karma-runner/karma/blob/master/lib/launcher.js#L142) with 
```
TypeError: Cannot call method 'then' of undefined
``` 

We can use asyc [kill-event](https://github.com/karma-runner/karma/blob/master/lib/launchers/base.js#L50) to kill the browser instead of overriding .kill() method.