emacs: 25.2 -> 25.3

https://lists.gnu.org/archive/html/emacs-devel/2017-09/msg00211.html

> This is an emergency release to fix a security vulnerability in Emacs.
>
> Enriched Text mode has its support for decoding 'x-display' disabled.
> This feature allows saving 'display' properties as part of text.
> Emacs 'display' properties support evaluation of arbitrary Lisp forms
> as part of instantiating the property, so decoding 'x-display' is
> vulnerable to executing arbitrary malicious Lisp code included in the
> text (e.g., sent as part of an email message).

pkgs/applications/editors/emacs/default.nix

@@ -26,12 +26,12 @@ let
 in
 stdenv.mkDerivation rec {
   name = "emacs-${version}${versionModifier}";
-  version = "25.2";
+  version = "25.3";
   versionModifier = "";
 
   src = fetchurl {
     url = "mirror://gnu/emacs/${name}.tar.xz";
-    sha256 = "1ykkq0xl28ljdg61bm6gzy04ww86ajms98gix72qg6cpr6a53dar";
+    sha256 = "02y00y9q42g1iqgz5qhmsja75hwxd88yrn9zp14lanay0zkwafi5";
   };
 
   patches = (lib.optional stdenv.isDarwin ./at-fdcwd.patch);