Use local gitter image

Author: Munter

docs/index.md

@@ -4,7 +4,7 @@ title: 'Mocha - the fun, simple, flexible JavaScript test framework'
 ---
 Mocha is a feature-rich JavaScript test framework running on [Node.js](http://nodejs.org) and in the browser, making asynchronous testing *simple* and *fun*. Mocha tests run serially, allowing for flexible and accurate reporting, while mapping uncaught exceptions to the correct test cases. Hosted on [GitHub](https://github.com/mochajs/mocha).
 
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