Update README.md

Try referencing th non-travis build status.
diff --git a/README.md b/README.md
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@@ -1,5 +1,5 @@
--[![Build Status](https://travis-ci.org/lrhn/charcode.svg?branch=master)](https://travis-ci.org/lrhn/charcode)
--[![Pub](https://img.shields.io/pub/v/charcode.svg)](https://pub.dev/packages/charcode) -
+[![Build Status](https://github.com/lrhn/charcode/workflows/Dart%20CI/badge.svg)](https://github.com/lrhn/charcode/actions?query=workflow%3A"Dart+CI")
+[![Pub](https://img.shields.io/pub/v/charcode.svg)](https://pub.dev/packages/charcode)
 
 # Character code constants