ncurses6: 20171125 -> 6.1

From my experience on slightly older tree,
a few *old* programs incorrectly rely on ncurses internal details
and may now require setting NCURSES_INTERNALS=1 to allow this badness.

Since this is release, we can grab it from gnu mirrors.

pkgs/development/libraries/ncurses/default.nix

@@ -11,18 +11,12 @@
 }:
 
 stdenv.mkDerivation rec {
-  version = "6.0-20171125";
+  version = "6.1";
   name = "ncurses-${version}" + lib.optionalString (abiVersion == "5") "-abi5-compat";
 
   src = fetchurl {
-    urls = [
-      # Remove this mirror on next upgrade, it's only needed because upstream took ncurses-6.0-20171125.tgz down!
-      "http://bld1.alpinelinux.org/distfiles/v3.5/ncurses-${version}.tgz"
-
-      "ftp://ftp.invisible-island.net/ncurses/current/ncurses-${version}.tgz"
-      "https://invisible-mirror.net/archives/ncurses/current/ncurses-${version}.tgz"
-    ];
-    sha256 = "11adzj0k82nlgpfrflabvqn2m7fmhp2y6pd7ivmapynxqb9vvb92";
+    url = "mirror://gnu/ncurses/${name}.tar.gz";
+    sha256 = "05qdmbmrrn88ii9f66rkcmcyzp1kb1ymkx7g040lfkd1nkp7w1da";
   };
 
   patches = lib.optional (!stdenv.cc.isClang) ./clang.patch;