Point CircleCi badge at master

Author: dehora

README.md

@@ -4,8 +4,7 @@ Beta PHP bindings for the Intercom API (https://api.intercom.io).
 
 [API Documentation](https://api.intercom.io/docs)
 
-[![Build Status](https://circleci.com/gh/intercom/intercom-php.png)](https://circleci.com/gh/intercom/intercom-php)
-
+[![Build Status](https://circleci.com/gh/intercom/intercom-php/tree/master.png)](https://circleci.com/gh/intercom/intercom-php/tree/master)
 
 ## Installation