[plt-scheme] Version Check

This is a view from the inside:

I consider DrScheme an operating system that you run atop your favorite 
machine-based OS.

Ethics: In the spirit of modern commercial OSes, I consider it 
perfectly okay to go out on the Web occasionally and check whether 
there are patches.

Technical: On my own hardware OS, I can ignore the dialog that says 
"new stuff is out there." That's critical. Sometimes I want it to hang 
around for a couple of hours, until I am ready. Sometimes I want to 
update now. Sometimes I want it to just go away for a long time. -- A 
modal dialog just doesn't cut it.

-- Matthias