Bump minimum rust version to 1.21

Needed for `From<&[T]> for Arc<T>`

Author: Markus Westerlind

.travis.yml

@@ -2,7 +2,7 @@ dist: trusty
 sudo: required
 language: rust
 rust:
-- 1.20.0
+- 1.21.0
 - stable
 - beta
 - nightly

README.md

@@ -208,7 +208,7 @@ recommended for general use.
 
 ### Minimum Rust version policy
 
-This crate's minimum supported `rustc` version is `1.20.0`.
+This crate's minimum supported `rustc` version is `1.21.0`.
 
 The current **tentative** policy is that the minimum Rust version required to
 use this crate can be increased in minor version updates. For example, if