Make coverage badge flat

README.md

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 # Zeppelin Solidity
 [![NPM Package](https://img.shields.io/npm/v/zeppelin-solidity.svg?style=flat-square)](https://www.npmjs.org/package/zeppelin-solidity)
 [![Build Status](https://img.shields.io/travis/OpenZeppelin/zeppelin-solidity.svg?branch=master&style=flat-square)](https://travis-ci.org/OpenZeppelin/zeppelin-solidity)
-[![Coverage Status](https://coveralls.io/repos/github/OpenZeppelin/zeppelin-solidity/badge.svg?branch=master)](https://coveralls.io/github/OpenZeppelin/zeppelin-solidity?branch=master)
+[![Coveralls github branch](https://img.shields.io/coveralls/github/OpenZeppelin/zeppelin-solidity/master.svg?style=flat-square)](https://coveralls.io/github/OpenZeppelin/zeppelin-solidity?branch=master)
 
 OpenZeppelin is a library for writing secure [Smart Contracts](https://en.wikipedia.org/wiki/Smart_contract) on Ethereum.