README: Update badges for new GitHub Actions workflows.

Signed-off-by: Damien George <damien@micropython.org>

README.md

-[![Build Status](https://travis-ci.com/micropython/micropython.png?branch=master)](https://travis-ci.com/micropython/micropython) [![Coverage Status](https://coveralls.io/repos/micropython/micropython/badge.png?branch=master)](https://coveralls.io/r/micropython/micropython?branch=master)
+[![CI badge](https://github.com/micropython/micropython/workflows/unix%20port/badge.svg)](https://github.com/micropython/micropython/actions?query=branch%3Amaster+event%3Apush) [![Coverage badge](https://coveralls.io/repos/micropython/micropython/badge.png?branch=master)](https://coveralls.io/r/micropython/micropython?branch=master)
 
 The MicroPython project
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