Update badges to show status of branch 6.x

README.md

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-[![Build status](https://ci.appveyor.com/api/projects/status/m7s53wav1l0abssg/branch/master?svg=true)](https://ci.appveyor.com/project/bblanchon/arduinojson/branch/master) [![Build Status](https://travis-ci.org/bblanchon/ArduinoJson.svg?branch=master)](https://travis-ci.org/bblanchon/ArduinoJson) [![Coverage Status](https://img.shields.io/coveralls/bblanchon/ArduinoJson.svg)](https://coveralls.io/r/bblanchon/ArduinoJson?branch=master) [![Star this project](http://githubbadges.com/star.svg?user=bblanchon&repo=ArduinoJson&style=flat&color=fff&background=007ec6)](https://github.com/bblanchon/ArduinoJson)
+[![Build status](https://ci.appveyor.com/api/projects/status/m7s53wav1l0abssg/branch/6.x?svg=true)](https://ci.appveyor.com/project/bblanchon/arduinojson/branch/6.x) [![Build Status](https://travis-ci.org/bblanchon/ArduinoJson.svg?branch=6.x)](https://travis-ci.org/bblanchon/ArduinoJson) [![Coverage Status](https://img.shields.io/coveralls/bblanchon/ArduinoJson.svg)](https://coveralls.io/r/bblanchon/ArduinoJson?branch=6.x) [![Star this project](http://githubbadges.com/star.svg?user=bblanchon&repo=ArduinoJson&style=flat&color=fff&background=007ec6)](https://github.com/bblanchon/ArduinoJson)
 
 ArduinoJson is a C++ JSON library for Arduino and IoT (Internet Of Things).