Update README.md

Co-Authored-By: Pavel Pustovalov <pustovalov.pavel@gmail.com>

README.md

@@ -4,7 +4,7 @@
 
 _Simple, scalable state management_
 
-[![CircleCI](https://circleci.com/gh/mobxjs/mobx.svg?style=svg)](https://circleci.com/gh/mobxjs/mobx)
+[![CircleCI](https://circleci.com/gh/mobxjs/mobx/tree/mobx4-master.svg?style=svg)](https://circleci.com/gh/mobxjs/mobx/tree/mobx4-master)
 [![Coverage Status](https://coveralls.io/repos/mobxjs/mobx/badge.svg?branch=master&service=github)](https://coveralls.io/github/mobxjs/mobx?branch=master)
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