PHPunit version update

Author: mianzaid

composer.json

@@ -41,7 +41,7 @@
 		"php": ">=7.0"
 	},
 	"require-dev": {
-		"phpunit/phpunit": "^4.8 || ^5.7 || ^7.5.15"
+		"phpunit/phpunit": ">=5"
 	},
 	"suggest": {
 		"ext-gd": "to use GD library based image processing."