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