Generated lock files contain different versions than nixpkgs

[l0b0]

To reproduce in this branch:

1. rm Gemfile.lock gemset.nix
2. bundix --lock

3. Compare

   $ nix-store --query --references "$(nix-instantiate shell.nix)" | grep 'jekyll-[0-9]'
   /nix/store/r8xwb2jyv16ch2qfn66fmanzybigy8dc-jekyll-4.2.0.drv

   with

   $ grep 'jekyll ([0-9]' Gemfile.lock
      jekyll (4.3.1)

Is it possible to get bundix to use the package version in the current shell in the lock files?

Attempted workaround: Manually set the correct gem version in Gemfile, as in, gem 'jekyll', '4.2.0'.