always clear the cache when updating a wagn+discussion

I have added this to our update script for the wagn.org server.  automating it for everyone doesn't seem worth the engineering effort at this point.  I added to Upgrade Notes under the Migration section.  John, it would be awesome if you get a chance to review that section-- several things listed there are automated now.

  --Lewis Hoffman.....Mon Jul 28 09:14:32 -0700 2008