redis-dump: use gemdir

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

···
   name = "redis-dump-0.3.5";
 
   inherit ruby;
-  gemfile = ./Gemfile;
-  lockfile = ./Gemfile.lock;
-  gemset = ./gemset.nix;
+  gemdir = ./.;
 
   buildInputs = [ perl autoconf ];