Update Rack gem to 2.1.3 or 2.2.0

Hakiri says there is a vulnerability in Rack:

[CVE-2020-8161] Directory traversal in Rack::Directory

Directory traversal in Rack::Directory

There was a possible directory traversal vulnerability in the Rack::Directory app
that is bundled with Rack.
This vulnerability has been assigned the CVE identifier CVE-2020-8161.

Versions Affected: rack < 2.2.0
Not affected: Applications that do not use Rack::Directory.
Fixed Versions: 2.1.3, >= 2.2.0

Testing-wise, if the site loads, we’re probably okay.