Switch core crate to rust 2018 edition
The http-client crate was already converted as part of
The win-users crate will be converted in a separate commit
[package]
name = "habitat_core"
version = "0.0.0"
+edition = "2018"
authors = ["Adam Jacob <adam@chef.io>", "Jamie Winsor <reset@chef.io>", "Fletcher Nichol <fnichol@chef.io>", "Joshua Timberman <joshua@chef.io>", "Dave Parfitt <dparfitt@chef.io>", "Steven Murawski <smurawski@chef.io>"]
workspace = "../../"
build = "build.rs"
