updated readme with new coverage badge
<a href="https://travis-ci.com/VirtualPlanetaryLaboratory/vplanet"><img src="https://travis-ci.com/VirtualPlanetaryLaboratory/vplanet.svg?token=jABaFLLgJNHTWSqkT7CM&branch=master"></a>
<a href="tests"><img src = "https://img.shields.io/badge/tests-46-darkorange.svg?style=flat"></a>
<img src = "https://img.shields.io/badge/memcheck-dirty-red.svg?style=flat">
- <a href="https://codecov.io/gh/VirtualPlanetaryLaboratory/vplanet">
- <img src="https://codecov.io/gh/VirtualPlanetaryLaboratory/vplanet/branch/master/graph/badge.svg" />
- </a>
+ <a href='https://coveralls.io/github/VirtualPlanetaryLaboratory/vplanet?branch=coverall'><img src='https://coveralls.io/repos/github/VirtualPlanetaryLaboratory/vplanet/badge.svg?branch=coverall' alt='Coverage Status' /></a>
</p>
### Overview
