Replace the coveralls badge by the codecov one

Author: dunglas

README.md

@@ -11,7 +11,7 @@ high performance API-first projects. Extend or override everything you want.
 
 [![Build Status](https://travis-ci.org/api-platform/core.svg?branch=master)](https://travis-ci.org/api-platform/core)
 [![Build status](https://ci.appveyor.com/api/projects/status/grwuyprts3wdqx5l?svg=true)](https://ci.appveyor.com/project/dunglas/dunglasapibundle)
-[![Coverage Status](https://coveralls.io/repos/github/api-platform/core/badge.svg)](https://coveralls.io/github/api-platform/core)
+[![codecov](https://codecov.io/gh/api-platform/core/branch/master/graph/badge.svg)](https://codecov.io/gh/api-platform/core)
 [![SymfonyInsight](https://insight.symfony.com/projects/92d78899-946c-4282-89a3-ac92344f9a93/mini.svg)](https://insight.symfony.com/projects/92d78899-946c-4282-89a3-ac92344f9a93)
 [![Scrutinizer Code Quality](https://scrutinizer-ci.com/g/api-platform/core/badges/quality-score.png?b=master)](https://scrutinizer-ci.com/g/api-platform/core/?branch=master)