README: Add Travis build status token

Author: TSC21

README.md

@@ -1,7 +1,7 @@
 MAVROS
 ======
 
-[![Gitter](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/mavlink/mavros?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+[![Gitter](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/mavlink/mavros?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)  [![Build Status](https://travis-ci.org/mavlink/mavros.svg?branch=master)](https://travis-ci.org/mavlink/mavros)
 
 MAVLink extendable communication node for ROS.