9pfs: add version

pkgs/tools/filesystems/9pfs/default.nix

···
 { lib, stdenv, fetchFromGitHub, fuse }:
 
 stdenv.mkDerivation rec {
-  name = "9pfs";
+  name = "9pfs-20150918";
 
   src = fetchFromGitHub {
     owner = "mischief";