Make README point to correct Travis CI Build Badge

The legacy branch should point to the legacy branch's
build status.

README.md

 ## noVNC: HTML5 VNC Client
 
-[![Build Status](https://travis-ci.org/kanaka/noVNC.svg?branch=master)](https://travis-ci.org/kanaka/noVNC)
+[![Build Status](https://travis-ci.org/kanaka/noVNC.svg?branch=novnc-legacy)](https://travis-ci.org/kanaka/noVNC)
 
 ### Description