Change badge for CI to GitHub Actions. (#2676)

* Change badge for CI to GitHub Actions.

* Add link to actions.

* Remove event=push

README.md

@@ -1,6 +1,6 @@
 # Computer Vision Annotation Tool (CVAT)
 
-[![Build Status](https://travis-ci.org/openvinotoolkit/cvat.svg?branch=develop)](https://travis-ci.org/openvinotoolkit/cvat)
+[![CI](https://github.com/openvinotoolkit/cvat/workflows/CI/badge.svg?branch=develop)](https://github.com/openvinotoolkit/cvat/actions)
 [![Codacy Badge](https://api.codacy.com/project/badge/Grade/b9899c72f2764df0b5d26390cb872e21)](https://app.codacy.com/gh/openvinotoolkit/cvat?utm_source=github.com&utm_medium=referral&utm_content=openvinotoolkit/cvat&utm_campaign=Badge_Grade_Dashboard)
 [![Gitter chat](https://badges.gitter.im/opencv-cvat/gitter.png)](https://gitter.im/opencv-cvat)
 [![Coverage Status](https://coveralls.io/repos/github/openvinotoolkit/cvat/badge.svg?branch=develop)](https://coveralls.io/github/openvinotoolkit/cvat?branch=develop)