Don't recurse on files, just the sub-dirs

We'll hit a 5K GitHub API limit (per hour) if we update more than a few times per hour, or if the number of projects in jsDelivr goes up. The update is slow now (could increase the limit on `async.eachLimit`, or lower the GitHub timeout), but it works for now.

A commit hook on the jsdelivr repo might work better to build the JSON then, rather than having api-sync build it on-demand.