replace ping6 with ping

reason: after the upgrade of iputils from 20151218 to 20161105
functionality of ping6 and tracepath6 was merged into ping and tracepath.

Ping is now mostly a drop-in replacment for ping6, except that selecting a
specific interface is done by encoding it into the address (ex.: fe80::1%eth0)
rather then specifing it with the `-I` flag.

nixos/modules/tasks/network-interfaces.nix

@@ -940,14 +940,8 @@ in
         source  = "${pkgs.iputils.out}/bin/ping";
         capabilities = "cap_net_raw+p";
       };
-
-      ping6 = {
-        source  = "${pkgs.iputils.out}/bin/ping6";
-        capabilities = "cap_net_raw+p";
-      };
     } else {
       ping.source = "${pkgs.iputils.out}/bin/ping";
-      "ping6".source = "${pkgs.iputils.out}/bin/ping6";
     };
 
     # Set the host and domain names in the activation script.  Don't