Update gecko-packages/gecko-php-unit requirement || ^3.0

Updates the requirements on [gecko-packages/gecko-php-unit](https://github.com/GeckoPackages/GeckoPHPUnit) to permit the latest version.
- [Release notes](https://github.com/GeckoPackages/GeckoPHPUnit/releases)
- [Changelog](https://github.com/GeckoPackages/GeckoPHPUnit/blob/3.0/CHANGELOG.md)
- [Commits](GeckoPackages/GeckoPHPUnit@v2.0...v3.1.1)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Author: dependabot-preview[bot]

composer.json

@@ -7,7 +7,7 @@
         "doctrine/inflector": "^1.0"
     },
     "require-dev": {
-        "gecko-packages/gecko-php-unit": "^2.0",
+        "gecko-packages/gecko-php-unit": "^2.0 || ^3.0",
         "phpunit/phpunit": "^6.0",
         "friendsofphp/php-cs-fixer": "2.14.2"
     },