Use the badge for GitHub Actions instead of Travis CI or buildkite

[tnir]

Goal

Test results should be visible for everyone.

Design

• Follows up PRs #574 and #737.

Changeset

• Change the badge image from Travis CI to GitHub Actions.
• Change the link from Travis CI to GitHub Actions (test-package).

Testing

• [x] is visible.
• [x] Having an access to https://github.com/bugsnag/bugsnag-ruby/actions/workflows/test-package.yml?query=branch%3Amaster

[johnkiely1]

Hi @tnir.

Thanks for the PR, we will review when priorities allow.

[tnir]

@imjoehaines @johnkiely1 Any updates?