Version 1.6.0

Release notes:

* The `union` feature is now compatible with stable Rust 1.50 (servo#248).
* The `const_generics` feature is now compatible with Rust 1.51-beta
  (servo#242).
* Fixed warnings when compiling with Rust 1.51-beta (servo#246).

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "smallvec"
-version = "1.5.1"
+version = "1.6.0"
 edition = "2018"
 authors = ["The Servo Project Developers"]
 license = "MIT/Apache-2.0"