Update readme's "master" to "main" for badges

Signed-off-by: Ivan Kozlovic <ivan@synadia.com>

README.md

@@ -3,8 +3,7 @@ A [Go](http://golang.org) client for the [NATS messaging system](https://nats.io
 
 [![License Apache 2](https://img.shields.io/badge/License-Apache2-blue.svg)](https://www.apache.org/licenses/LICENSE-2.0)
 [![FOSSA Status](https://app.fossa.io/api/projects/git%2Bgithub.com%2Fnats-io%2Fgo-nats.svg?type=shield)](https://app.fossa.io/projects/git%2Bgithub.com%2Fnats-io%2Fgo-nats?ref=badge_shield)
-[![Go Report Card](https://goreportcard.com/badge/github.com/nats-io/nats.go)](https://goreportcard.com/report/github.com/nats-io/nats.go) [![Build Status](https://travis-ci.com/nats-io/nats.go.svg?branch=master)](http://travis-ci.com/nats-io/nats.go) [![GoDoc](https://img.shields.io/badge/GoDoc-reference-007d9c)](https://pkg.go.dev/github.com/nats-io/nats.go)
- [![Coverage Status](https://coveralls.io/repos/nats-io/nats.go/badge.svg?branch=master)](https://coveralls.io/r/nats-io/nats.go?branch=master)
+[![Go Report Card](https://goreportcard.com/badge/github.com/nats-io/nats.go)](https://goreportcard.com/report/github.com/nats-io/nats.go) [![Build Status](https://travis-ci.com/nats-io/nats.go.svg?branch=main)](https://travis-ci.com/nats-io/nats.go) [![GoDoc](https://img.shields.io/badge/GoDoc-reference-007d9c)](https://pkg.go.dev/github.com/nats-io/nats.go) [![Coverage Status](https://coveralls.io/repos/nats-io/nats.go/badge.svg?branch=main)](https://coveralls.io/r/nats-io/nats.go?branch=main)
 
 ## Installation