docs: remove coverage badge (non existent)
<p align="center">
<a href="https://travis-ci.org/libp2p/go-libp2p"><img src="https://travis-ci.org/libp2p/go-libp2p.svg?branch=master" /></a>
<!--<a href="https://circleci.com/gh/libp2p/go-libp2p"><img src="https://circleci.com/gh/libp2p/go-libp2p.svg?style=svg" /></a>-->
- <a href="https://coveralls.io/github/libp2p/go-libp2p?branch=master"><img src="https://coveralls.io/repos/github/libp2p/go-libp2p/badge.svg?branch=master"></a>
+ <!--<a href="https://coveralls.io/github/libp2p/go-libp2p?branch=master"><img src="https://coveralls.io/repos/github/libp2p/go-libp2p/badge.svg?branch=master"></a>-->
<br>
<a href="https://github.com/RichardLitt/standard-readme"><img src="https://img.shields.io/badge/standard--readme-OK-green.svg?style=flat-square" /></a>
<a href="https://godoc.org/github.com/libp2p/go-libp2p"><img src="https://godoc.org/github.com/ipfs/go-libp2p?status.svg" /></a>
