Add $queue attribute

This code enables me to change the queue name that the JobPipeline will be pushed to.

Now I can do something like this:
```php
JobPipeline::make([
    Jobs\CreateDatabase::class,
    Jobs\MigrateDatabase::class,
    // Jobs\SeedDatabase::class,

    // Your own jobs to prepare the tenant.
    // Provision API keys, create S3 buckets, anything you want!

])->send(function (Events\TenantCreated $event)
{
    return $event->tenant;
})->onQueue('another-queue')
    ->shouldBeQueued()
```

src/JobPipeline.php

@@ -27,6 +27,9 @@ class JobPipeline implements ShouldQueue
     /** @var bool */
     public $shouldBeQueued;
     
+    /** @var string */
+    public $queue;
+
     public function __construct($jobs, callable $send = null, bool $shouldBeQueued = null)
     {
         $this->jobs = $jobs;
@@ -57,6 +60,13 @@ class JobPipeline implements ShouldQueue
         return $this;
     }
     
+    public function onQueue(string $queue)
+    {
+        $this->queue = $queue;
+
+        return $this;
+    }
+
     public function handle(): void
     {
         foreach ($this->jobs as $job) {