cluster.disconnect() does worker.disconnect() for us

fixes:
events.js:72
        throw er; // Unhandled 'error' event
              ^
Error: IPC channel is already disconnected
    at process.target.disconnect (child_process.js:476:26)
    at ProgressTracker.callback (cluster.js:473:20)
    at ProgressTracker.check (cluster.js:94:32)
    at ProgressTracker.done (cluster.js:91:8)
    at Object.2:3 (cluster.js:490:20)
    at handleResponse (cluster.js:171:41)
    at respond (cluster.js:192:5)
    at handleMessage (cluster.js:202:5)
    at process.EventEmitter.emit (events.js:117:20)
    at handleMessage (child_process.js:318:10)
fix Gruntfile
release 1.0.2