Update PHPUnit to highest compatible (#24)

Author: Ekman

composer.json

@@ -36,7 +36,7 @@
 		"php": "^7.0"
 	},
 	"require-dev": {
-		"phpunit/phpunit": "^6.0",
+		"phpunit/phpunit": "^6.0||^7.0",
 		"friendsofphp/php-cs-fixer": "^2.15",
 		"php-coveralls/php-coveralls": "^2.1"
 	},