doc: Replace travis with CircleCI badge

Author: estahn

README.md

@@ -3,7 +3,7 @@
 [![Latest Stable Version](https://poser.pugx.org/estahn/phpunit-json-assertions/version.png)](https://packagist.org/packages/estahn/phpunit-json-assertions)
 [![Total Downloads](https://poser.pugx.org/estahn/phpunit-json-assertions/d/total.png)](https://packagist.org/packages/estahn/phpunit-json-assertions)
 [![SensioLabsInsight](https://insight.sensiolabs.com/projects/581c093b-833a-49c2-a05c-d99aaf8f39c2/mini.png)](https://insight.sensiolabs.com/projects/581c093b-833a-49c2-a05c-d99aaf8f39c2)
-[![Build Status](https://travis-ci.org/estahn/phpunit-json-assertions.png?branch=master)](https://travis-ci.org/estahn/phpunit-json-assertions)
+[![CircleCI](https://circleci.com/gh/estahn/phpunit-json-assertions.svg?style=svg)](https://circleci.com/gh/estahn/phpunit-json-assertions)
 [![StyleCI](https://styleci.io/repos/53177096/shield)](https://styleci.io/repos/53177096)
 [![Average time to resolve an issue](http://isitmaintained.com/badge/resolution/estahn/phpunit-json-assertions.svg)](http://isitmaintained.com/project/estahn/phpunit-json-assertions "Average time to resolve an issue")
 [![Percentage of issues still open](http://isitmaintained.com/badge/open/estahn/phpunit-json-assertions.svg)](http://isitmaintained.com/project/estahn/phpunit-json-assertions "Percentage of issues still open")