Updated PHPUnit - allow to use 5, 6 and 7 series

Author: michalbundyra

composer.json

@@ -32,7 +32,7 @@
         "zendframework/zend-stdlib": "^2.7 || ^3.0"
     },
     "require-dev": {
-        "phpunit/PHPUnit": "^4.5",
+        "phpunit/phpunit": "^5.7.27 || ^6.5.8 || ^7.1.4",
         "zendframework/zend-coding-standard": "~1.0.0"
     },
     "conflict": {