adds codecov badge to README.md

README.md

 *NOTE: This README reflects the in development version of Mirador 3. For previous versions, please reference that release's README directly. Latest 2.x release: [v.2.7.0](https://github.com/ProjectMirador/mirador/tree/v2.7.0)*
 # Mirador
-[![Build Status](https://travis-ci.org/ProjectMirador/mirador.svg?branch=master)](https://travis-ci.org/ProjectMirador/mirador) [![Test Coverage](https://api.codeclimate.com/v1/badges/85265abbd7ce99df2d90/test_coverage)](https://codeclimate.com/github/ProjectMirador/mirador/test_coverage)
+[![Build Status](https://travis-ci.org/ProjectMirador/mirador.svg?branch=master)](https://travis-ci.org/ProjectMirador/mirador) [![codecov](https://codecov.io/gh/ProjectMirador/mirador/branch/master/graph/badge.svg)](https://codecov.io/gh/ProjectMirador/mirador)
 
 ## Running Mirador locally