message: Add Build image to README
diff: -# harp.gl
  +# harp.gl [![Build Status](https://travis-ci.com/heremaps/harp.gl.svg?token=XqJjRxFbW25Pc73LNRB9&branch=master)](https://travis-ci.com/heremaps/harp.gl)
  `harp.gl` is an _experimental and work in progress_ open-source 3D map rendering engine.
  