ruby-modules: Add new url `source` type

This just defers to `fetchurl` for fetching.

Additionally, update the `nix-bundle-install.rb` script to handle gems
installed from a path, i.e. those with a `url` source.
Some parts of that script have been disabled in the `path` case
that likely shouldn't be, but cause errors and aren't necessary to get
`vagrant` to work.