Fix build badge

Author: tvdijen

README.md

@@ -1,4 +1,4 @@
 xml-security
 ===========================
-[![Build Status](https://travis-ci.org/simplesamlphp/xml-security.svg?branch=master)](https://travis-ci.org/simplesamlphp/xml-security)
+[![Build Status](https://github.com/simplesamlphp/xml-security/workflows/CI/badge.svg?branch=master)](https://github.com/simplesamlphp/xml-security/actions)
 [![Coverage Status](https://codecov.io/gh/simplesamlphp/xml-security/branch/master/graph/badge.svg)](https://codecov.io/gh/simplesamlphp/xml-security)