Add cron:update rake task and whenever gem to run it. Close #9.

For each period, if it has been updated:
- 30<t minutes ago: always update
- 15<t<30 minutes ago: sometimes update (1/3 of the time)
- t<15 minutes ago: never update

The purpose for the randomness is to minimize the chance that all
of the periods get synced up and update at the same time.