Keep Box token refreshed

[sbeitzel]

If we go for too long an interval between updates, the authorization token for rclone to Box expires. This often happens toward mid-season, when we have settled all the tunes and aren't doing any more tweaks. The next time we update the tune repo, the tunes build but the synchronization to Box fails.

The way to deal with this would be to perform a heartbeat sync once or twice a day, so that the token will get renewed when the expiration time gets close.