chore(core): update to 2018 edition

Author: tottoto

headers-core/Cargo.toml

@@ -8,6 +8,7 @@ homepage = "https://hyper.rs"
 repository = "https://github.com/hyperium/headers"
 authors = ["Sean McArthur <sean@seanmonstar.com>"]
 keywords = ["http", "headers", "hyper", "hyperium"]
+edition = "2018"
 rust-version = "1.49"
 
 [dependencies]

headers-core/src/lib.rs

@@ -8,8 +8,6 @@
 //! This is the core crate of the typed HTTP headers system, providing only
 //! the relevant traits. All actual header implementations are in other crates.
 
-extern crate http;
-
 pub use http::header::{self, HeaderName, HeaderValue};
 
 use std::error;