Update README's header section (mochajs#2214)

Improves the looks of the README's top/header section:

- Updates logo to use the new design, svg hosted on Clodup
- Center logo
- Gives logo some top/bottom margin

README.md

@@ -1,11 +1,13 @@
+<br><br>
+<p align="center">
+  <img src="https://cldup.com/xFVFxOioAU.svg" alt="Mocha test framework"/>
+</p>
+<br><br>
+
 [![Build Status](https://api.travis-ci.org/mochajs/mocha.svg?branch=master)](http://travis-ci.org/mochajs/mocha) [![Gitter](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/mochajs/mocha?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 [![OpenCollective](https://opencollective.com/mochajs/backers/badge.svg)](#backers)
 [![OpenCollective](https://opencollective.com/mochajs/sponsors/badge.svg)](#sponsors)
 
-
-
-  [![Mocha test framework](http://f.cl.ly/items/3l1k0n2A1U3M1I1L210p/Screen%20Shot%202012-02-24%20at%202.21.43%20PM.png)](http://mochajs.org)
-
   Mocha is a simple, flexible, fun JavaScript test framework for node.js and the browser. For more information view the [documentation](http://mochajs.org).
 
 ## Links