Update Rust and Elixir dependencies and change compatibility

[mischov]

Freshen up dependencies and change compatibility guarantees to reflect Rustler's (3 latest versions of Elixir, 3 latest versions of Erlang/OTP).

• Update to latest versions of the html5ever family of libraries
• Update to Rustler 0.27
• Use Rust 2018 edition
• Rust format and Clippy fix
• Update compatibility to reflect Rustler's compatibility guarantees
• Update test workflow