Update coverage badge

README.md

@@ -1,5 +1,5 @@
 [![Build Status](https://snap-ci.com/ratpack/ratpack/branch/master/build_image)](https://snap-ci.com/ratpack/ratpack/branch/master)
-[![Coverage Status](https://img.shields.io/coveralls/ratpack/ratpack.svg)](https://coveralls.io/r/ratpack/ratpack)
+[![codecov.io](http://codecov.io/github/ratpack/ratpack/coverage.svg?branch=master)](http://codecov.io/github/ratpack/ratpack?branch=master)
 
 [![Ratpack.io](/ratpack-site/src/assets/images/ratpack-logo.png?raw=true)](https://ratpack.io)