Sync README.md with .travis.yml says [ci skip]

Ruby 2.1 is being tested.

Author: David Rodríguez

README.md

@@ -100,6 +100,7 @@ VCR has been tested on the following ruby interpreters:
 * MRI 1.9.2
 * MRI 1.9.3
 * MRI 2.0.0
+* MRI 2.1
 * REE 1.8.7
 * JRuby
 * Rubinius