attribute ‘cn-outline’ missing on darwin (emacs)

[matthewbauer]

Issue description

Running this gives me an error:

$ nix-env --arg config.allowBroken false -f. -qaA emacsPackagesNg.melpaPackages
error: attribute ‘cn-outline’ missing, at /Users/mbauer/Projects/nixpkgs/pkgs/applications/editors/emacs-modes/melpa-packages.nix:54:31

Seems to be an issue with the broken override not working in this case?

/cc @ttuegel @mdorman

Technical details

• System: (NixOS: nixos-version, Ubuntu/Fedora: lsb_release -a, ...)
  macOS

• Nix version: (run nix-env --version)
  nix-env (Nix) 1.11.15

• Nixpkgs version: (run nix-instantiate --eval '<nixpkgs>' -A lib.nixpkgsVersion)
  18.03pre119383.eafd703a63

• Sandboxing enabled: (run grep build-use-sandbox /etc/nix/nix.conf)
  no

[mdorman]

Ah, so cn-outline was removed from melpa, but there was a hand-written override left over. Will address. @matthewbauer since you're a member of the org, could you assign this to me?