[Rivet] rivet 1.8.1

[Mikhail KIRSANOV]

  It is installed in GENSER in HepMC trees 2.03.11, 2.06.05 and 2.06.08

  I decided to move it to the main GENSER generators table

  http://sftweb.cern.ch/generators/home

  There, one can see the versions of auxiliary packages in the README 
link (for this reason for this time I put the rivet list on copy)

  The installation with fastjet 3 will follow.

                                         GENSER team