Use queue to serialize jobs

I'll wait for it to be merged on SlmQueue before updating tests.
This won't work; you have to pass on the complete serialized + JSON string. Now you are decoding the JSON, but you don't need to. The API is

```php
public function unserializeJob($string, array $metadata = array())
```

Where `$string` is the payload from the queue and `$metadata` is additional metadata you want to hydrate. In your case:

```php
$jobs[] = $this->unserializeJob($message, array(
    'id'            => $message['MessageId'],
    'receiptHandle' => $message['ReceiptHandle'],
    'md5'           => $message['MD5OfBody']
));
```

The idea was to completely remove the format of JSON in the queue adapters :)