Add edition to Cargo.toml and fix description typo

Edition added as 2018. 2015 throws errors.
Version is not changed.

Author: allan2

Cargo.toml

@@ -2,11 +2,12 @@
 name = "rustc-hash"
 version = "1.0.1"
 authors = ["The Rust Project Developers"]
-description = "speed, non-cryptographic hash used in rustc"
+description = "A speedy, non-cryptographic hashing algorithm used by rustc and Firefox"
 license = "Apache-2.0/MIT"
 readme = "README.md"
 keywords = ["hash", "fxhash", "rustc"]
 repository = "https://github.com/rust-lang-nursery/rustc-hash"
+edition = "2018"
 
 [features]
 std = []