release.nix: disable builds with ghc 7.9.x
release.nix: disable builds with ghc 7.9.x

ghc-head is broken in Nixpkgs because it changes the way the package
database is organized.

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