message: README: update coveralls badge
diff: [![jenkins build status](http://ci.px4.io:8080/buildStatus/icon?job=DronecodeSDK/develop)](http://ci.px4.io:8080/blue/organizations/jenkins/DronecodeSDK/activity)
  [![travis-ci build status](https://travis-ci.org/Dronecode/DronecodeSDK.svg?branch=develop)](https://travis-ci.org/Dronecode/DronecodeSDK)
  [![appveyor build status](https://ci.appveyor.com/api/projects/status/1ntjvooywpxmoir8/branch/develop?svg=true)](https://ci.appveyor.com/project/julianoes/dronecore/branch/develop)
  -[![Coverage Status](https://coveralls.io/repos/github/dronecore/DroneCore/badge.svg?branch=develop)](https://coveralls.io/github/dronecore/DroneCore?branch=develop)
  +[![Coverage Status](https://coveralls.io/repos/github/Dronecode/DronecodeSDK/badge.svg?branch=develop)](https://coveralls.io/github/Dronecode/DronecodeSDK?branch=develop)
  ## Description
  