Update build status label to show Github Actions
This replaces Travis-CI badge which is no longer in use.
## OpenFaaS &reg; - Serverless Functions Made Simple
-[![Build Status](https://travis-ci.com/openfaas/faas.svg?branch=master)](https://travis-ci.com/openfaas/faas)
+[![Build Status](https://github.com/openfaas/faas/actions/workflows/build.yml/badge.svg)](https://travis-ci.com/openfaas/faas)
[![go.dev reference](https://img.shields.io/badge/go.dev-reference-007d9c?logo=go&logoColor=white&style=flat-square)](https://pkg.go.dev/github.com/openfaas/faas)
[![License: MIT](https://img.shields.io/badge/License-MIT-yellow.svg)](https://opensource.org/licenses/MIT)
[![OpenFaaS](https://img.shields.io/badge/openfaas-serverless-blue.svg)](https://www.openfaas.com)
