more accurate string representation of RedisCluster object

With the test cluster in vagrant, a string representation of the object now looks like:

```
RedisCluster<127.0.0.1:7000,127.0.0.1:7001,127.0.0.1:7002,127.0.0.1:7003,127.0.0.1:7004,127.0.0.1:7005>
```