Fixed build status badge (#2086)

Project moved from Travis CI configuration e9d46bfe

README.md

 ![OAuth2 Proxy](/docs/static/img/logos/OAuth2_Proxy_horizontal.svg)
 
-[![Build Status](https://secure.travis-ci.org/oauth2-proxy/oauth2-proxy.svg?branch=master)](http://travis-ci.org/oauth2-proxy/oauth2-proxy)
+[![Continuous Integration](https://github.com/oauth2-proxy/oauth2-proxy/actions/workflows/ci.yaml/badge.svg)](https://github.com/oauth2-proxy/oauth2-proxy/actions/workflows/ci.yaml)
 [![Go Report Card](https://goreportcard.com/badge/github.com/oauth2-proxy/oauth2-proxy)](https://goreportcard.com/report/github.com/oauth2-proxy/oauth2-proxy)
 [![GoDoc](https://godoc.org/github.com/oauth2-proxy/oauth2-proxy?status.svg)](https://godoc.org/github.com/oauth2-proxy/oauth2-proxy)
 [![MIT licensed](https://img.shields.io/badge/license-MIT-blue.svg)](./LICENSE)