Update build status image in README

Author: weavejester

README.md

@@ -1,6 +1,6 @@
 # Ring-JSON
 
-[![Build Status](https://secure.travis-ci.org/ring-clojure/ring-json.png)](http://travis-ci.org/ring-clojure/ring-json)
+[![Build Status](https://travis-ci.org/ring-clojure/ring-json.svg?branch=master)](https://travis-ci.org/ring-clojure/ring-json)
 
 Standard Ring middleware functions for handling JSON requests and
 responses.