Improve CI

Author: VincentLanglet

composer.json

@@ -19,7 +19,7 @@
 		"jakub-onderka/php-parallel-lint": "^1.0",
 		"phing/phing": "^2.16.0",
 		"phpstan/phpstan-strict-rules": "^0.12",
-		"phpunit/phpunit": "^7.0",
+		"phpunit/phpunit": "^7.0 || ^8.0 || ^9.0",
 		"satooshi/php-coveralls": "^1.0",
 		"slevomat/coding-standard": "^4.7.2"
 	},