Use SVG version of "Deploy" button

Author: jdanford

README.md

@@ -35,7 +35,7 @@ $ heroku open
 ```
 or
 
-[![Deploy](https://www.herokucdn.com/deploy/button.png)](https://heroku.com/deploy)
+[![Deploy](https://www.herokucdn.com/deploy/button.svg)](https://heroku.com/deploy)
 
 ## Documentation