Update to Rust edition 2018

[timvisee]

This PR updates the project to Rust edition 2018.

The Travis CI build currently fails due to missing dependencies in CI. This is fixed separately in #73 .

[mcobzarenco]

I found myself doing the same thing (+ running cargo fmt) as groundwork for adding Send bounds to the Error types -- now I'm wondering if this project is still actively maintained?

[timvisee]

I'm afraid it isn't. The fork copypasta is actively maintained though.

[mcobzarenco]

Thanks for pointing that out 💯 - I was in the process of making making a fork myself!