message: serialize version directly
diff: @@ -54,7 +54,7 @@ impl PartialOrd for Package {
  #[derive(Serialize)]
  struct SerializedPackage<'a> {
  name: &'a str,
  - version: &'a str,
  + version: &'a Version,
  id: PackageId,
  license: Option<&'a str>,
  license_file: Option<&'a str>,
  @@ -104,7 +104,7 @@ impl ser::Serialize for Package {
  SerializedPackage {
  name: &*package_id.name(),
  - version: &package_id.version().to_string(),
  + version: &package_id.version(),
  id: package_id,
  license,
  license_file,
  