Simplify nix integration a bit

default.nix

@@ -1,5 +0,0 @@
-with import <nixpkgs> {};
-stdenv.mkDerivation {
-    name = "a-robots-conundrum";
-    buildInputs = [ (python27.withPackages (ps: with ps; [ pygame numpy ])) ];
-}

shell.nix

@@ -0,0 +1,6 @@
+# Use this file with nix-shell or similar tools; see https://nixos.org/
+with import <nixpkgs> {};
+
+mkShell {
+  buildInputs = [ (python27.withPackages (ps: with ps; [ pygame numpy ])) ];
+}