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

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`.
