message: DOC: fix badges for RTK CI
diff: RTK: The Reconstruction ToolKit
  -------------------------------
  -.. image:: https://circleci.com/gh/SimonRit/RTK/tree/RTK-ExternalModule.svg?style=shield
  +.. image:: https://circleci.com/gh/SimonRit/RTK/tree/master.svg?style=shield
  :target: https://circleci.com/gh/SimonRit/RTK
  -.. image:: https://travis-ci.org/SimonRit/RTK.svg?branch=RTK-ExternalModule
  +.. image:: https://api.travis-ci.org/SimonRit/RTK.svg
  :target: https://travis-ci.org/SimonRit/RTK/branches
  -.. image:: https://ci.appveyor.com/api/projects/status/dpflok54d64t3dq9/branch/RTK-ExternalModule?svg=true
  +.. image:: https://ci.appveyor.com/api/projects/status/dpflok54d64t3dq9/branch/master?svg=true
  :target: https://ci.appveyor.com/project/simonrit/rtk
  Links
  