Motivation for this change

Django 2_2 is already up to date (see #66046). This PR updates older versions of django supported in nixpkgs.

See release notes:

• https://docs.djangoproject.com/en/1.11/releases/1.11.23/
• https://docs.djangoproject.com/en/2.1/releases/2.1.11/

Once merged, I’ll handle back porting to release-19.03.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

Notify maintainers

cc @