readme: update

apparently [`redisStore()`](https://github.com/koajs/koa-redis) no longer requires the `new` keyword to be used, so this PR updates the readme to reflect that change. Thanks!
Merge pull request #30 from yoshuawuyts/patch-1

readme: update
Delete session-id cookie if session not loaded.

To defend against session fixation attacks, delete the session-id cookie if the
store load fails.