[plt-scheme] Version Check

I think phoning home is a terrible idea.  What people run on their own
machines is nobody's business.  My guess is that most people won't
mind, but from a few I think you'll get some pretty angry backlash
about it.  And since you are, in fact, well-meaning folks, you won't
be accustomed to the abuse.  It's not pleasant having people attribute
nasty motives to you when you had nothing of the sort.

There are so many alternatives:
- Add a button to the splash screen that says, "check for updates".
- Once a week (or whatever), pop up a dialog that suggests checking
for updates.  That could also have options for "don't suggest checking
for updates again" "suggest again later", etc.
- Add a "check for updates" item to the File menu, or someplace more prominent.

Why expose yourself to the abuse by doing it automatically?