[Rivet] Hepforge upgrade: new Rivet URL

[David Grellscheid]

Dear Rivet admins,

As part of the Hepforge upgrade project, we have moved your web pages to 
their new URL http://rivet.hepforge.org/

Any old-style URL should be redirected to the right place.

The Trac system has been upgraded to Trac 12, but all other Hepforge 
services continue unchanged for now.

Please let us know any inconsistencies or problems that you notice.

Thanks!

   David