Remove durable config from the readme

[LucasRoesler]

Context

In #95 the durable configuration was removed

https://github.com/openfaas/nats-queue-worker/pull/95/files#diff-3511d0be867c6e99793319238ebe2438L48

So we should remove this line from the README

https://github.com/openfaas/nats-queue-worker/blame/master/README.md#L37

[alexellis]

@matthiashanel could you take this please?

[alexellis]

See also: openfaas/docs#225