Update to modern Rust

[riptl]

A few changes I needed

• Target InfluxDB 1.7.9
• Use Rust 2018
• Use futures-0.3 and async/.await
• Bump url to 2.1
• Replace hyper with reqwest
• Remove tokio dependency (only used for tests)
• Apply cargo clippy suggestions

Checklist

• [x] cargo test passes
• [x] cargo clippy clean
• [x] Fix Travis build

Fixes #25

[riptl]

@slackve I merged upstream master and bumped some dependencies. Please merge asap!