Update Travis-CI build status image

Author: opengeek

README.md

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 # MODX Revolution
 
-[![Build Status](https://travis-ci.org/modxcms/revolution.svg?branch=2.x)](https://travis-ci.org/modxcms/revolution) [![Slack Chat](https://img.shields.io/badge/chat_in_slack-online-green.svg?longCache=true&style=flat&logo=slack)](https://modx.org)
+[![Build Status](https://travis-ci.com/modxcms/revolution.svg?branch=3.x)](https://travis-ci.org/modxcms/revolution) [![Slack Chat](https://img.shields.io/badge/chat_in_slack-online-green.svg?longCache=true&style=flat&logo=slack)](https://modx.org)
 
 ## Content Management System and Application Framework