Motivation for this change

Also specified specific revisions of the file to download to not have
a breaking package whenever the author decides to update the package.

This happened on 2018-01-01 when the author updated the copyright of
the package to match the new year.

I have not managed to build it with nox, nox promptly told me that there was no change, which I don't agree to. And I've also been unable to build it with nix-build like this:

nix-build -A emacsPackagesNgGen.font-lock-plus

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.