add fallback PHPUnit 5.7.x for tests against PHP 5.6

Author: frankfoerster

composer.json

@@ -22,7 +22,7 @@
   },
   "require-dev": {
     "cakephp/cakephp": "3.*",
-    "phpunit/phpunit": "6.*",
+    "phpunit/phpunit": "^5.7|^6.0",
     "cakephp/cakephp-codesniffer": "3.*"
   },
   "autoload": {