Migrate the tests from Travis CI to GitHub Actions

[PeterDaveHello]

Resolve #150

Migrate the tests from Travis CI to GitHub Actions.

• README.md:

  • Remove Travis CI badge.
  • Add GitHub Actions badge for tests.

• .travis.yml:

  • Delete the file as it is no longer needed.

• .github/workflows/tests.yml:

  • Create a new GitHub Actions workflow file.
  • Define jobs for PHP versions 7.1, 7.2, 7.3, and 7.4.
  • Include steps to set up PHP, install dependencies, and run tests.

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[PeterDaveHello]

I'll rewrite it 😅