how to cabal-install with ghc-8.rc3 ?

Hi. I would love to test rc3. Building (with 7.10.3) from source went fine. But then I cannot cabal-install anything: random-1.1: installed package info from too old version of Cabal (key field does not match id field) Failed to install random-1.1 What is the recommended upgrade path? ( bootstrap.sh in cabal/cabal-install (current from github) does not work, tries to download a Cabal version that's not on hackage ) Thanks, J.W.