message: README: added travis status badge
diff: # DroneCore
  +[![Build Status](https://travis-ci.org/dronecore/DroneCore.svg?branch=master)](https://travis-ci.org/dronecore/DroneCore)
  +
  ## Description
  DroneCore is an API and library for the [PX4 flight stack](http://github.com/PX4/Firmware) using [Mavlink](http://mavlink.org).
  