Update CI badge from Travis to Github

Commit 728bf9d changed from Travis to
GitHub Actions, but the badge displayed on the README and the link that
it's using still point to travis, so we always display information from
an old build.

This commit addresses the issue by updating the badge links to point to
GitHub.

README.md

@@ -1,6 +1,6 @@
 # Clearance
 
-[![Build Status](https://secure.travis-ci.org/thoughtbot/clearance.svg)](http://travis-ci.org/thoughtbot/clearance?branch=master)
+[![Build Status](https://github.com/thoughtbot/clearance/actions/workflows/tests.yml/badge.svg)]( https://github.com/thoughtbot/clearance/actions/workflows/tests.yml?query=branch%3Amain)
 [![Code Climate](https://codeclimate.com/github/thoughtbot/clearance.svg)](https://codeclimate.com/github/thoughtbot/clearance)
 [![Documentation Quality](https://inch-ci.org/github/thoughtbot/clearance.svg?branch=main)](https://inch-ci.org/github/thoughtbot/clearance)
 [![Reviewed by Hound](https://img.shields.io/badge/Reviewed_by-Hound-8E64B0.svg)](https://houndci.com)