Update Botan unstable and fix source link

pkgs/development/libraries/botan/generic.nix

···
 
   src = fetchurl {
     name = "Botan-${version}.tgz";
-    url = "http://files.randombit.net/botan/v${baseVersion}/Botan-${version}.tgz";
+    urls = [
+       "http://files.randombit.net/botan/v${baseVersion}/Botan-${version}.tgz"
+       "http://botan.randombit.net/releases/Botan-${version}.tgz"
+    ];
     inherit sha256;
   };
 

pkgs/development/libraries/botan/unstable.nix

···
 
 callPackage ./generic.nix (args // {
   baseVersion = "1.11";
-  revision = "14";
-  sha256 = "1laa6d8w9v39a2pfmilj62jwc67r0jbq5f3xdlffd3kvkdnwcysb";
+  revision = "15";
+  sha256 = "1rkv84v09llbxyvh33szi7zsjm19l02j7h60n9g7jhhc2w667jk0";
   openssl = null;
 })