buildRubyGem: use a saner default version to fix #11805

Previously the gems defaulted to "ruby" as the name and
"${ruby-version}-${gem-name}-${gem-version}" as the version,
which was just insane.

https://github.com/NixOS/nixpkgs/issues/9771#issuecomment-141041414
Noone is reacting so it's high time to take at least some action.
/cc @cstrahan.