Fix local testing and update tests

[raulcodes]

Local testing

• Moves away from using worldpeaceio/wordpress-integration image for integration testing since it is no longer maintained.
• Adds dockerfile and scripts for setting up a testing environment.
• Updates phpunit configuration
• Fixes deprecations in tests
• Updates dev dependencies
• Adds plugin update ability

[ivnflpz]

Verified the tests run locally and that I can use GeoIP short codes in a local build.

Have some minor comments but otherwise looks good.

[ivnflpz]

LGTM!