`usesGemspec` no longer required to trigger the "copy everything into
gemfile-and-lock" behavior. If the mainGem is referred to by path,
that's sufficient.
Add `pname` attribute: if passed, the derivation name defaults to the
gem name + version and only expose the gem's bin.
Add `gemdir` attribute: gives a default lookup path for the Gemfile,
Gemfile.lock and gemset.nix.
Set the `meta.platforms' to `ruby.meta.platforms' by default.
