Ruby minor version issue

shell.nix

let
  pkgs = import <nixpkgs> {}
  {
    inherit overlays;
  };
  # unstable = import (fetchTarball https://nixos.org/channels/nixos-unstable/nixexprs.tar.xz) { };
  stdenv = pkgs.stdenv;

  ruby = pkgs.ruby_2_4;
  rubygems = (pkgs.rubygems.override { ruby = ruby; });

in stdenv.mkDerivation rec {
  name = "name";
  buildInputs = [
    ruby
    pkgs.git
    pkgs.bundix
  ];
}