prerelease.keybase.io should keep releases for greater than 2 days

I went to upgrade my Ubuntu packages today (20180816) and ran `sudo apt upgrade`. The upgrade failed because keybase failed with this error

> E: Failed to fetch http://prerelease.keybase.io/deb/pool/main/k/keybase/keybase_2.6.0-20180814163904.23e84dff6f_amd64.deb 403 Forbidden [IP: 13.35.121.24 80]

This is because the package from 2 days ago (20180814) wasn't the absolute newest keybase package and prerelease.keybase.io is returning 403 forbidden for anything but the newest package.

I worked around this by running `sudo apt update` then `sudo apt upgrade` which fetched a package that was released today (`2.6.0-20180815194228.604cfa0d29`).

Please change the release retention period on prerelease.keybase.io to some value greater than 2 days. 2 days is a silly short amount of time to retain releases for. This way users won't have their upgrades blocked by this kind of error.

Also, if the problem is that the release from 2 days prior has been deleted, serve up a `404` instead of a `403` to avoid confusing users (as is happened in #13142)

Start by reviewing the prerelease.keybase.io package retention behavior and the apt upgrade failure described for the 20180814 package. Done means packages remain fetchable for more than two days, or expired packages return 404 rather than 403; compare the related behavior in issue #13142.

Written by the indexing model from the issue text.