Update to 2018 edition

[CosmicHorrorDev]

This updates things to fall more in line with post-2018-edition rust along with some small cleanup

I believe that I avoided any unnecessary breaking changes. The only difficult part was dealing with TryInto<reqwest::Url, Error = crate::Error> which worked when using the library's TryInto implementation, but can't work with std's because of the orphan rule. Switching to reqwest::Url should accept the same values and still can bubble the error up to a crate::Error though

I also removed the #![cfg_attr(test, deny(warnings))] since it's a bit annoying for my workflow where I generally use a program to watch the source files and run tests when it sees a file changed (using bacon while another good alternative is cargo-watch). It's a bit cumbersome to not be able to easily differentiate between warnings and errors when working like this and I don't think it's a big issue since warnings are denied in CI

[CosmicHorrorDev]

I decided to temporarily allow warnings in CI until the deprecation notices from using error_chain get sorted out

[compressed]

LGTM, thank you.

[compressed]

Also agree on the #![cfg_attr(test, deny(warnings))] point. I use cargo-watch heavily so I can see how this would be annoying. The intent was there before, but not needed now.