Replace sha256 in gemset.nix with hash and use SRI hashes

[fabaff]

Inside Nixpkgs the migration to hash with a SRI hash was startet a while back. To be constant it would be nice if bundix could support this as well.

  actionview = {
    groups = ["default"];
    platforms = [];
    source = {
      remotes = ["https://rubygems.org"];
      sha256 = "0xnpdwj1d8m6c2d90jp9cs50ggiz0jj02ls2h9lg68k4k8mnjbd2";
[...]

to

  actionview = {
    groups = ["default"];
    platforms = [];
    source = {
      remotes = ["https://rubygems.org"];
      hash = "sha256-wBqw+YHKlxYplgsYL1pbkusHyPfCaVPcH/Yku6WDYbE=";
[...]