Update download backend stubs to 2018 edition
Update download backend stubs to 2018 edition

- fixes #1789

inejge/rustup.rs,rust-lang/rustup,rust-lang/rustup,nodakai/rustup.rs,inejge/rustup.rs,inejge/rustup.rs,nodakai/rustup.rs,nodakai/rustup.rs,nodakai/rustup.rs,inejge/rustup.rs,nodakai/rustup.rs,inejge/rustup.rs,rust-lang/rustup,rust-lang/rustup,rust-lang/rustup