Travis icon in README.md: show build status of the current master rather than...

Travis icon in README.md: show build status of the current master rather than latest build status (can be any pull request)

README.md

 The Contiki Operating System
 ============================
 
-[![Build Status](https://secure.travis-ci.org/contiki-os/contiki.png)](http://travis-ci.org/contiki-os/contiki)
+[![Build Status](https://travis-ci.org/contiki-os/contiki.svg?branch=master)](https://travis-ci.org/contiki-os/contiki/branches)
 
 Contiki is an open source operating system that runs on tiny low-power
 microcontrollers and makes it possible to develop applications that