Replace badge to CircleCI (#7216)

README.md

 ![Mastodon](https://i.imgur.com/NhZc40l.png)
 ========
 
-[![Build Status](https://img.shields.io/travis/tootsuite/mastodon.svg)][travis]
+[![Build Status](https://img.shields.io/circleci/project/github/tootsuite/mastodon.svg)][circleci]
 [![Code Climate](https://img.shields.io/codeclimate/maintainability/tootsuite/mastodon.svg)][code_climate]
 
-[travis]: https://travis-ci.org/tootsuite/mastodon
+[circleci]: https://circleci.com/gh/tootsuite/mastodon
 [code_climate]: https://codeclimate.com/github/tootsuite/mastodon
 
 Mastodon is a **free, open-source social network server** based on **open web protocols** like ActivityPub and OStatus. The social focus of the project is a viable decentralized alternative to commercial social media silos that returns the control of the content distribution channels to the people. The technical focus of the project is a good user interface, a clean REST API for 3rd party apps and robust anti-abuse tools.