ocamlPackages.js_of_ocaml: 3.0.0 -> 3.1.0

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

- built on NixOS
/tmp/tmp.I0xNTw0Lrg /home/ryantm/.cache/nixpkgs
/home/ryantm/.cache/nixpkgs
- 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 3.1.0 with grep in /nix/store/rg3dh55klmmy5gffvv353nbgwjg0mh11-js_of_ocaml-3.1.0
- directory tree listing: https://gist.github.com/4afbe9caa678f6e8124185a1cc5d2a9c