samba: 4.7.6 -> 4.7.9

Version 4.7.9 is a security release. The versions 4.7.7 & 4.7.8 contain
numerous bug fixes.

This fixes a couple of security related issues:
 - https://www.samba.org/samba/security/CVE-2018-10858.html
 - https://www.samba.org/samba/security/CVE-2018-10918.html
 - https://www.samba.org/samba/security/CVE-2018-10919.html
 - https://www.samba.org/samba/security/CVE-2018-1139.html

Changelogs for the version upgrades can be found below.

 - https://www.samba.org/samba/history/samba-4.7.7.html
 - https://www.samba.org/samba/history/samba-4.7.8.html
 - https://www.samba.org/samba/history/samba-4.7.9.html

pkgs/servers/samba/4.x.nix

@@ -22,11 +22,11 @@ with lib;
 
 stdenv.mkDerivation rec {
   name = "samba-${version}";
-  version = "4.7.6";
+  version = "4.7.9";
 
   src = fetchurl {
     url = "mirror://samba/pub/samba/stable/${name}.tar.gz";
-    sha256 = "0vkxqp3wh7bpn1fd45lznmrpn2ma1fq75yq28vi08rggr07y7v8y";
+    sha256 = "1v0pd2k4rfdzcqbzb3g5gjiy8rwqamppwzwy5swz4x5rxyr5567c";
   };
 
   outputs = [ "out" "dev" "man" ];