Remove mentions of Travis, update the build status badge

Github Actions is the CI now, update the README accordingly

Author: Jussi Kukkonen

.travis.yml

@@ -3,7 +3,6 @@ include LICENSE
 
 # Add test config files to show how to run tests
 include tox.ini
-include .travis.yml
 include requirements*.txt
 
 # Include all files under the tests directory (including test data)

MANIFEST.in

@@ -1,8 +1,8 @@
 Secure Systems Library
 ----------------------
 
-.. image:: https://travis-ci.com/secure-systems-lab/securesystemslib.svg?branch=master
-   :target: https://travis-ci.com/secure-systems-lab/securesystemslib
+.. image:: https://github.com/securesystemslab/securesystemslib/workflows/Run%20Securesystemslib%20tests/badge.svg
+   :target: https://github.com/securesystemslab/securesystemslib/actions?query=workflow%3A%22Run+TUF+tests%22+branch%3Amaster
 
 .. image:: https://coveralls.io/repos/github/secure-systems-lab/securesystemslib/badge.svg?branch=master
    :target: https://coveralls.io/github/secure-systems-lab/securesystemslib?branch=master