fix: launcher kill method was throwing an error if no callback was specified

bug introduced in https://github.com/karma-runner/karma/commit/8647266fd592fe245aaf2be964319d3026432e33

In server.js on the browser_complete event no callback is passed to the launcher.kill method 
So for example if you have 2 browsers and one finish before the other one was even connected karma dies with a TypeErrror.
TypeError: undefined is not a function
    at process._tickCallback (node.js:415:13)