crashed when calling `client.end()` from `pubsub` client

[ghostoy]

The cause was in that this.pubClient and this.subClient were undefined. See
https://github.com/strongloop/strong-pubsub-redis/blob/master/index.js#L73-L74.

Looks like these variables were not assigned in constructor or should they be changed to this.redisPubClient and this.redisSubClient.