Update composer.json

Author: prolic

composer.json

@@ -20,7 +20,7 @@
         "zendframework/zend-diactoros": "^1.1.3"
     },
     "require-dev": {
-        "phpunit/phpunit": "^6.6",
+        "phpunit/phpunit": "^6.5",
         "mikey179/vfsStream": "^1.6.4",
         "slim/slim": "^3.0",
         "zendframework/zend-expressive": "^3.0",