Only enable the redis brain if REDISTOGO_URL is defined

This adds a new `graceful-redis-brain.coffee` script, which uses the redis brain if `REDISTOGO_URL` is explicitly defined; otherwise it does nothing. This allows developers to hack on hubot without having to install redis, while allowing redis to be easily enabled if needed.

If a developer wants to use their local instance of redis, they can set `REDISTOGO_URL` to `redis://localhost:6379`. (Previously, they needed to do nothing, but then hubot would have complained if the local redis server couldn't be contacted.)