freetds: fix broken url (close #9031)

pkgs/development/libraries/freetds/default.nix

···
 assert odbcSupport -> unixODBC != null;
 
 stdenv.mkDerivation rec {
-  name = "freetds-0.91.112";
+  name = "freetds-0.91";
 
   src = fetchurl {
-    url = "ftp://ftp.astron.com/pub/freetds/stable/${name}.tar.gz";
-    sha256 = "be4f04ee57328c32e7e7cd7e2e1483e535071cec6101e46b9dd15b857c5078ed";
+    url = "http://mirrors.ibiblio.org/freetds/stable/${name}.tar.gz";
+    sha256 = "0r946axzxs0czsmr7283w7vmk5jx3jnxxc32d2ncxsrsh2yli0ba";
   };
 
   buildInputs = stdenv.lib.optional odbcSupport [ unixODBC ];
···
     platforms = stdenv.lib.platforms.all;
   };
 }
-