nlohmann_json: 3.1.1 -> 3.1.2

Semi-automatic update generated by https://github.com/ryantm/nix-update tools. These checks were done:

- built on NixOS
- Warning: no binary found that responded to help or version flags. (This warning appears even if the package isn't expected to have binaries.)
- found 3.1.2 with grep in /nix/store/ak7p93w4pkmjvx5fi82fg40rra5z6g44-nlohmann_json-3.1.2
- directory tree listing: https://gist.github.com/cebf8a5968a9c2ae65ec58227044fcad

pkgs/development/libraries/nlohmann_json/default.nix

@@ -4,13 +4,13 @@
 
 stdenv.mkDerivation rec {
   name = "nlohmann_json-${version}";
-  version = "3.1.1";
+  version = "3.1.2";
 
   src = fetchFromGitHub {
     owner = "nlohmann";
     repo = "json";
     rev = "v${version}";
-    sha256 = "0s5xiyvnvxc2k0zkyb12mm5cwn61lavyxlfpknlx5f243g1xi6f6";
+    sha256 = "1mpr781fb2dfbyscrr7nil75lkxsazg4wkm749168lcf2ksrrbfi";
   };
 
   nativeBuildInputs = [ cmake ];