Added support for PHPUnit 9.

Add support for PHPUnit 9 (#3)

* Updated unit tests to support both version 8 and 9 of PHPUnit.
 * Refactored GitHub Actions to run the tests in both PHPUnit 8 and 9.

Added support for PHPUnit 8

Add support for PHPUnit 8 (#2)

 * Allowed PHPUnit 8 since the interface hasn't been changed.
 * Replaced Travis CI with GitHub Actions.

Flexible traits implementation

• Refactored the original abstract class to a more flexible trait solution.
• Added configuration for which extended classes to sniff and auto fix.

First release of strict unit tests

• Added StrictTestCase that fails when an unexpected method call is made in a test.
• Added code sniffer functionality.