cabal2nix.cabal: advertise the fact that we can build with ghc 6.12.3
cabal2nix.cabal: advertise the fact that we can build with ghc 6.12.3

gridaphobe/cabal2nix,jb55/cabal2nix,spencerjanssen/cabal2nix,dmalikov/cabal2nix,psibi/cabal2nix,Fuuzetsu/cabal2nix,bennofs/cabal2nix