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

@@ -210,7 +210,7 @@ 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