Modernize edition, bump deps, fix traits

[clux]

From my fork that now contains 3 different changes:

• error handling now Sync with failure

• reqwest 0.10 with url 2 for better url parsing
• drop TryFrom traits where they conflict with stdlib
• rust 2018 everywhere

• allow picking reqwest tls backend with features

The first was when dealing with slack-hook in a setting where we needed the errors to be shareable between actix. Error chain is not great for that, but there are other alternatives. failure was just an easy one. We never had any need for more than rudimentary error handling in this library anyway.

Feel free to cherry-pick or review. I do realise this project has not received much maintenance the last year, so for now will just publish a fork via a branch in my upstream. But putting this PR here as a just in case :heart: