Update to latest auth0-PHP dependency

[evansims]

This PR;

• Bumps the specified Auth0-PHP SDK dependency to 7.9, addressing a Guzzle deprecation
• Fixes an issue with Auth0ServiceTest and PSR-4
• Updates the CircleCI configuration to a same format used by the PHP SDK repo
• Updates coding standard rules to the same format used by the PHP SDK repo
• Performs some minor code clean-up to bring it in line with the updated coding standards
• Adds a docker-compose.yml to run tests containerized
• Adds some new details to composer.json for discoverability on Packagist, and simplifies the script commands