message: use to_string instead of format
diff: @@ -379,8 +379,8 @@ fn check_for_duplicate_names(manifest: &Manifest) -> Result<(), failure::Error>
  .collect::<Vec<String>>()
  .join(", ");
  let duplicate_message = match duplicate_names.len() {
  - 1 => Some(format!("this name is duplicated")),
  - n if n >= 2 => Some(format!("these names are duplicated")),
  + 1 => Some("this name is duplicated".to_string()),
  + n if n >= 2 => Some("these names are duplicated".to_string()),
  _ => None,
  };
  if let Some(message) = duplicate_message {
  