Use phpunit 5.7

Author: bcrowe

composer.json

@@ -20,7 +20,7 @@
         "cakephp/cakephp": "~3.4"
     },
     "require-dev": {
-        "phpunit/phpunit" : "~6.0",
+        "phpunit/phpunit" : "~5.7",
         "scrutinizer/ocular": "~1.1",
         "squizlabs/php_codesniffer": "~2.3.0"
     },