Fix for user data not persisting in brain

These changes (credit to @patcon) seem to at least roughly allow user data to be persisted, similar to how the Campfire adapter works.  

*NOTE the hubot-slack adapter also appears to depend on the latest version of hubot.  When upgrading, we ran into issues with some of our scripts, mainly because some methods have been moved from Robot to Brain.*  E.g.:
```
robot.userForId id
```
should instead be used like
```
robot.brain.userForId id
```