Don't Require Redis Cluster

[chriskinsman]

We have been using disque as a very simple queue on various projects and loved how simple it was to spin up a single instance and use it.

The requirement of spinning up a minimum three node redis cluster adds a lot of complexity especially in k8s based testing environments where the persistent volume support may not be present to enable this.

Please consider allowing it to work on a single node redis instance.

Thanks!