config.nix

{
  packageOverrides = pkgs:
  let
    bundlerGem = { buildRubyGem, makeWrapper, ruby, coreutils }:
    buildRubyGem {
      name = "bundler-1.9.10";
      namePrefix = "";
      sha256 = "0ygpfvk51x96a8r3jknx7vf6afabxqp8byayvkka3rqkwv0jssqs";
      dontPatchShebangs = true;
      postInstall = ''
        find $out -type f -perm -0100 | while read f; do
          substituteInPlace $f \
             --replace "/usr/bin/env" "${coreutils}/bin/env"
        done

        wrapProgram $out/bin/bundler \
          --prefix PATH ":" ${ruby}/bin
      '';
    };
  in
  rec {
    bundlerEnv = pkgs.bundlerEnv.override {
      bundler_HEAD = bundlerGem {
        buildRubyGem = pkgs.buildRubyGem;
        makeWrapper = pkgs.makeWrapper;
        ruby = pkgs.ruby;
        coreutils = pkgs.coreutils;
      };
    };
  };
}