Batch job does not terminate with Karma 1.4.0

### Expected behaviour

The batch job `gulp test_browser` should terminate when the Gulp task `test_browser` finishes.

### Actual behaviour

The batch job `gulp test_browser` does not terminate when the Gulp task `test_browser` finishes. I need to kill the job manually (`Ctrl + C`).

### Environment Details

- Operating system: Windows 10 Home
- Karma version (output of `karma --version`): 1.4.0

**gulpfile.js**

```javascript
gulp.task('test_browser', function(done) {
gutil.log(gutil.colors.yellow('Running tests in Google Chrome:'));

var server = new Server({
configFile: __dirname + '/karma.conf.js',
files: [
'test/js/helpers/**/*.js',
paths.dist_window + '/**/*.js',
'test/js/specs/**/*Spec.js'
]
}, done);

server.start();
});
```

### Steps to reproduce the behaviour

1. I upgraded from `"karma": "1.3.0"` to `"karma": "1.4.0"` (in `package.json`)
2. When I run `gulp test_browser`, then this job does not terminate with Karma 1.4.0 (but it does with Karma 1.3.0)
3. The project I used for testing is: https://github.com/wireapp/bazinga64

**Terminal output with Karma v1.3.0 server**


**Terminal output with Karma v1.4.0 server**


Please let me know if you need more details. Happy to help!

Start with gulpfile.js and package.json: compare the test_browser invocation and Karma 1.3.0 versus 1.4.0, then reproduce on Windows 10 with gulp test_browser. Use karma.conf.js and the linked bazinga64 project for configuration context. Done means the command exits on its own after the test_browser task finishes under Karma 1.4.0.

Written by the indexing model from the issue text.