Travis: Test on stable, nightly and 1.21

.travis.yml

@@ -1,4 +1,8 @@
 language: rust
+rust:
+  - 1.21.0
+  - stable
+  - nightly
 sudo: false
 script:
   - cargo build --verbose

README.md

@@ -6,6 +6,8 @@ as described in
 
 [Documentation](https://unicode-rs.github.io/unicode-normalization/unicode_normalization/index.html)
 
+This crate requires Rust 1.21+.
+
 ```rust
 extern crate unicode_normalization;