Fix gemset generation for Gem version including the platform requirement

[Morriar]

Some gems like sorbet-static or sys-proctable (< 1.2) compose their version number by appending the platform cpu, os and version.

For example, the following Gemfile:

source 'https://rubygems.org' do
  gem 'sorbet', '= 0.4.4821'
end

Gives the following Gemfile.lock:

GEM
  remote: https://rubygems.org/
  specs:
    sorbet (0.4.4821)
      sorbet-static (= 0.4.4821)
    sorbet-static (0.4.4821-universal-darwin-14)

PLATFORMS
  ruby

DEPENDENCIES
  sorbet (= 0.4.4821)!

BUNDLED WITH
   1.12.5

But will produce the following gemset.nix file with bundix:

{
  sorbet = {
    dependencies = ["sorbet-static"];
    groups = ["development" "test"];
    platforms = [];
    source = {
      remotes = ["https://rubygems.org"];
      sha256 = "099nvhim356rvg0rmx71lnr74fy0rbh6m51fzknwsb59dskn0nrg";
      type = "gem";
    };
    version = "0.4.4821";
  };
  sorbet-static = {
    groups = ["default" "development" "test"];
    platforms = [];
    source = {
      remotes = ["https://rubygems.org"];
      sha256 = "0v0ldhw7wqh9554l002lz7vzqjhkcqwhnr3k14c85fqwmrfixhxm";
      type = "gem";
    };
    version = "0.4.4821";
  };
}

Notice how the platform specified in the Gemfile.lock for sorbet-static is lost when the gemset.nix is generated. From 0.4.4821-universal-darwin-14 to 0.4.4821.

This is a problem since the package manager for NixOs only relies on the version attribute to download the required gem (code copied from https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/ruby-modules/gem/default.nix#L56-L62):

    if type == "gem" then
      fetchurl {
        urls = map (
          remote: "${remote}/gems/${gemName}-${version}.gem"
        ) (attrs.source.remotes or [ "https://rubygems.org" ]);
        inherit (attrs.source) sha256;
      }

This pull-request appends the platform to the compiled gemset.nix file so we get:

{
  sorbet-static = {
    groups = ["default" "development" "test"];
    platforms = [];
    source = {
      remotes = ["https://rubygems.org"];
      sha256 = "0v0ldhw7wqh9554l002lz7vzqjhkcqwhnr3k14c85fqwmrfixhxm";
      type = "gem";
    };
    version = "0.4.4834-universal-darwin-14";
  };
}

And the NixOS package manager is happy.

[Morriar]

Cargo culted from https://github.com/bundler/bundler/blob/fe5d24b5dc6614c0edfee8407acba14aa610ac72/lib/bundler/lazy_specification.rb#L54-L69.

[Morriar]

Closing this one in favor of #68 which handle the problem more gracefully.