Fix phpunit min ver.

Follow cakephp/cakephp's one

Author: o0h

composer.json

@@ -30,7 +30,7 @@
   },
   "require-dev": {
     "cakephp/cakephp": "^3.5",
-    "phpunit/phpunit": "^5.7"
+    "phpunit/phpunit": "^5.7.14|^6.0"
   },
   "autoload": {
     "psr-4": {