v5.0.0: Merge pull request #23 from share/redis-v4

BREAKING:

• #23 — only support redis>=4: See redis's migration guide for upgrading their driver. Once using the new driver version, sharedb-redis-pubsub should need minimal changes to its configuration: if passing in a redis client, it may be eagerly connected, or may not be (as before). If using a configuration object, check the new redis@4 configuration guide