Revert "Use coveralls for coverage badge (onosproject#257)"

This reverts commit 014da33.

README.md

@@ -2,7 +2,7 @@
 [![Build Status](https://travis-ci.org/onosproject/onos-config.svg?branch=master)](https://travis-ci.org/onosproject/onos-config)
 [![Go Report Card](https://goreportcard.com/badge/github.com/onosproject/onos-config)](https://goreportcard.com/report/github.com/onosproject/onos-config)
 [![License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](https://github.com/gojp/goreportcard/blob/master/LICENSE)
-[![Coverage Status](https://coveralls.io/repos/github/onosproject/onos-config/badge.svg?branch=master)](https://coveralls.io/github/onosproject/onos-config?branch=master)
+[![Coverage](https://img.shields.io/codecov/c/gh/onosproject/onos-config.svg)](https://codecov.io/gh/onosproject/onos-config)
 
 ONOS Configuration subsystem built using the µONOS architecture