Use GitHub Actions Rather than Travis

[grant]

For various reasons, we should use GitHub actions for CI rather than Travis:

Expected

• Use GitHub Actions
• No Travis config: https://github.com/GoogleCloudPlatform/functions-framework-php/blob/master/.travis.yml

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Here's an example GitHub Action workflow we can use:
https://github.com/googleapis/google-api-php-client/blob/master/.github/workflows/tests.yml