redis-dump: add redis-load

There's a companion script that will restore the dumped json.

pkgs/development/tools/redis-dump/default.nix

···
 bundlerApp {
   pname = "redis-dump";
   gemdir = ./.;
-  exes = [ "redis-dump" ];
+  exes = [ "redis-dump" "redis-load" ];
 
   passthru.updateScript = bundlerUpdateScript "redis-dump";