Bump minimum rustc version to 1.21.0 (#270)

Author: messense

.travis.yml

@@ -15,7 +15,7 @@ matrix:
           env: FEATURES=""
 
         # minimum version
-        - rust: 1.18.0
+        - rust: 1.21.0
 
         - rust: stable
           env: FEATURES="--features unstable"