Update crate for rust-postgres 0.17 & Rust 2018 syntax

[lfittl]

Thanks for the excellent library!

I've had a use case that required tstzrange integration, and thought it'd be useful to get this library updated for the latest rust-postgres 0.17 release:

• Updates integration and tests to postgres 0.17
• Moves chrono crate support to "with-chrono-0_4" to match postgres crate
• Drops time crate support since it was dropped in postgres crate
• Use Rust 2018 syntax & ensure code follows up-to-date idioms

[sfackler]

Thanks!