use PHPUnit ^5.3 as now we're supporting php5.6+

Author: eugene-matvejev

composer.json

@@ -30,6 +30,6 @@
     },
     "require-dev": {
         "composer/composer": "^1.1",
-        "phpunit/phpunit": "^4.8"
+        "phpunit/phpunit": "^5.3"
     }
 }