github-actions: update build badge

The following commit updates the `build badge` in the `readme` to
display the new github actions workflow, and status.

Signed-off-by: vince-fugnitto <vincent.fugnitto@ericsson.com>

README.md

@@ -10,7 +10,7 @@
   [![Gitpod Ready-to-Code](https://img.shields.io/badge/Gitpod-Ready--to--Code-blue?logo=gitpod)](https://gitpod.io/#https://github.com/eclipse-theia/theia)
   [![PRs Welcome](https://img.shields.io/badge/PRs-welcome-brightgreen.svg?style=flat-curved)](https://github.com/eclipse-theia/theia/labels/help%20wanted)
   [![Discourse status](https://img.shields.io/discourse/status?label=Chat&server=https%3A%2F%2Fcommunity.theia-ide.org%2F)](https://community.theia-ide.org/)
-  [![Build Status](https://travis-ci.com/eclipse-theia/theia.svg?branch=master)](https://travis-ci.com/eclipse-theia/theia)
+  [![Build Status](https://github.com/eclipse-theia/theia/workflows/Build/badge.svg?branch=master)](https://github.com/eclipse-theia/theia/actions?query=branch%3Amaster)
   [![Publish VS Code Built-in Extensions](https://github.com/theia-ide/vscode-builtin-extensions/workflows/publish-vscode-built-in-extensions/badge.svg)](https://github.com/theia-ide/vscode-builtin-extensions/actions)
   [![Open questions](https://img.shields.io/badge/Open-questions-blue.svg?style=flat-curved)](https://github.com/eclipse-theia/theia/labels/question)
   [![Open bugs](https://img.shields.io/badge/Open-bugs-red.svg?style=flat-curved)](https://github.com/eclipse-theia/theia/labels/bug)