Address CVE-2020-8161 in `rack` gem

[johnpaulashenfelter]

There is a CVE in the rack gem: https://groups.google.com/forum/#!topic/ruby-security-ann/T4ZIsfRf2eA

We cannot upgrade rack because of other dependencies on specific versions. We can't patch the library without forking and using our own package delivery through GitHub which is not a quick fix.

[johnpaulashenfelter]

Finally closed completely in department-of-veterans-affairs/vets-api#4499

[alexpappasoddball]

closing this because upgrade became possible with a patch release from sidekiq v5