[plt-scheme] scheduled PLaneT server downtime

Just a heads-up:

The PLaneT server will be down tomorrow, Tuesday May 7, from
approximately 2:00PM to approximately 2:30PM Chicago time, for
hardware upgrades and to  upgrade the underlying mzscheme installation
that runs the PLaneT server's software. During this time you won't be
able to visit the PLaneT web pages, or upload or retrieve packages.

Hopefully these upgrades will improve the server's response times and
fix some network-related issues.

-jacob