curl: 7.55.1 -> 7.56.0 for CVE-2017-1000254

https://curl.haxx.se/docs/adv_20171004.html
(cherry picked from commit a98b96824db90446895c7cbf2c4931ef9ad9cb68)

pkgs/tools/networking/curl/default.nix

···
 assert c-aresSupport -> c-ares != null;
 
 stdenv.mkDerivation rec {
-  name = "curl-7.55.1";
+  name = "curl-7.56.0";
 
   src = fetchurl {
     url = "http://curl.haxx.se/download/${name}.tar.bz2";
-    sha256 = "1yvcn7jbh99gsqhc040nky0h15a1cfh8yic6k0a1zhdhscpakcg5";
+    sha256 = "1pvr2bqfhk46bzq2x2hskmnq3wc8qxlps7apm9q1qiixb9ra8q6y";
   };
 
   outputs = [ "bin" "dev" "out" "man" "devdoc" ];