In building a gem whose native extension is a Rakefile, the previous
version of this code will call essentially `rake ""`, when it means to
call `rake`.
This change converts `""` into `[]` rather than `[""]`.
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.
If a gemspec has UTF-8 characters in it, ruby will fail loading it with
invalid multibyte char (US-ASCII)
This change forces the encoding to be correct, we assume everyone now
uses UTF-8.
