Server hangs during initialization when using redis as SessionStore

Hi,

connect-redis does not have a 'ready' callback like connect-mongo does, so when trying to use redis, the server hangs.

You can verify that [here](https://github.com/kcbanner/connect-mongo/blob/master/lib/connect-mongo.js#L63) and [here](https://github.com/tj/connect-redis/blob/master/lib/connect-redis.js#L42)

As the the promise is never fulfilled, the server waits forever.

Also this is my first pull request here, I just started messing around with Keystone :) So I'm not sure if I made any mistakes.