Correctly pass through callback parameter of task function to subtasks

Given a sub task expecting a callback function to signal task completion such as:

```javascript
gulp.task('foo-bar', function(cb) {
    doSomethingAsynchronous(cb);
});
```

This works when gulp is called form the subfolder, but when called from gulp-hub this will fail as the callback function cb is always undefined. This PR fixes it, passing through the callback function from its task definitions.