Fixed bug in `Parallel Processing With Cluster` example code in `Readme.md`.

The following example code from the [Parallel Processing With Cluster](https://github.com/LearnBoost/kue#parallel-processing-with-cluster) section of `Readme.md`

```js
setInterval(function(){
  job.log('sending!');
  job.progress(total - pending, total);
  --pending || done();
}, 1000);
```

keeps on firing this Redis message

```
"q:job:123:log" "sending!"
"hset" "q:job:123" "updated_at" "1372407738102"
"hset" "q:job:123" "progress" "100"
"hset" "q:job:123" "updated_at" "1372407738102"
"publish" "q:events" "{\"id\":\"123\",\"event\":\"progress\",\"args\":[\"progress\",100]}"
```

even after a job is done. `clearInterval` needs to be called as follows

```js
var interval = setInterval(function(){
  job.log('sending!');
  job.progress(total - pending, total);
  --pending || done();
  pending || clearInterval(interval);
}, 1000);
```