Update README.md to reflect current version.

BUG=
[email protected]

Review URL: https://codereview.appspot.com/201780043

Author: jacob314

README.md

@@ -16,7 +16,7 @@ Add Quiver to your project's pubspec.yaml file and run `pub get`.
 We recommend the following version constraint:
 
     dependencies:
-      quiver: '>=0.20.0<0.21.0'
+      quiver: '>=0.21.0<0.22.0'
 
 # Main Libraries