Update the badge to use GitHub Actions for CI (#641)

diff --git a/README.md b/README.md
index d34d074..6f41857 100644
--- a/README.md
+++ b/README.md
@@ -2,7 +2,7 @@
 
 A server backend to support DartPad.
 
-[![Build Status](https://travis-ci.org/dart-lang/dart-services.svg?branch=master)](https://travis-ci.org/dart-lang/dart-services)
+[![Build Status](https://github.com/dart-lang/dart-services/workflows/dart-services/badge.svg)](https://github.com/dart-lang/dart-services/actions?workflow=dart-services)
 [![Coverage Status](https://coveralls.io/repos/dart-lang/dart-services/badge.svg?branch=master)](https://coveralls.io/r/dart-lang/dart-services?branch=master)
 [![Uptime Status](https://img.shields.io/badge/uptime-Pingdom-blue.svg)](http://stats.pingdom.com/8n3tfpl1u0j9)
 [![Gitpod Ready-to-Code](https://img.shields.io/badge/Gitpod-Ready--to--Code-blue?logo=gitpod)](https://gitpod.io/#https://github.com/dart-lang/dart-services)