Correct Travis CI Badge URL

The readme was using the URL for the 'refactor/cleanup'
branch, which doesn't exist anymore.  It now uses
the badge for the master branch as it should.

README.md

@@ -1,6 +1,6 @@
 ## noVNC: HTML5 VNC Client
 
-[![Build Status](https://travis-ci.org/kanaka/noVNC.svg?branch=refactor%2Fcleanup)](https://travis-ci.org/kanaka/noVNC)
+[![Build Status](https://travis-ci.org/kanaka/noVNC.svg?branch=master)](https://travis-ci.org/kanaka/noVNC)
 
 ### Description