remove coveralls, add codecov, remove appveyor (#209)

README.md

@@ -18,8 +18,7 @@ limitations under the License.
 
 [![GHA](https://github.com/openzipkin/zipkin-go/actions/workflows/ci.yml/badge.svg?event=push)](https://github.com/openzipkin/zipkin-go/actions/workflows/ci.yml)
 [![CircleCI](https://circleci.com/gh/openzipkin/zipkin-go.svg?style=shield)](https://circleci.com/gh/openzipkin/zipkin-go)
-[![Appveyor CI](https://ci.appveyor.com/api/projects/status/1d0e5k96g10ajl63/branch/master?svg=true)](https://ci.appveyor.com/project/basvanbeek/zipkin-go)
-[![Coverage Status](https://img.shields.io/coveralls/github/openzipkin/zipkin-go.svg)](https://coveralls.io/github/openzipkin/zipkin-go?branch=master)
+[![codecov](https://codecov.io/gh/openzipkin/zipkin-go/branch/master/graph/badge.svg?token=gXdWofFlsq)](https://codecov.io/gh/openzipkin/zipkin-go)
 [![Go Report Card](https://goreportcard.com/badge/github.com/openzipkin/zipkin-go)](https://goreportcard.com/report/github.com/openzipkin/zipkin-go)
 [![GoDoc](https://godoc.org/github.com/openzipkin/zipkin-go?status.svg)](https://godoc.org/github.com/openzipkin/zipkin-go)
 [![Gitter chat](https://badges.gitter.im/openzipkin/zipkin.svg)](https://gitter.im/openzipkin/zipkin?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)