add GitHub Action badge; remove Travis scripts and configuration (#78)

Author: dgrove-oss

.travis.yml

@@ -20,7 +20,8 @@
 # Apache OpenWhisk runtimes for .NET
 
 [![License](https://img.shields.io/badge/license-Apache--2.0-blue.svg)](http://www.apache.org/licenses/LICENSE-2.0)
-[![Build Status](https://travis-ci.com/apache/openwhisk-runtime-dotnet.svg?branch=master)](https://travis-ci.com/github/apache/openwhisk-runtime-dotnet)
+[![Continuous Integration](https://github.com/apache/openwhisk-runtime-dotnet/actions/workflows/ci.yaml/badge.svg)](https://github.com/apache/openwhisk-runtime-dotnet/actions/workflows/ci.yaml)
+
 ## Give it a try today
 
 Create a C# project called Apache.OpenWhisk.Example.Dotnet: