chore(composer): try for php5.6

Author: adhocore

composer.json

@@ -27,7 +27,7 @@
         "ext-ctype": "*"
     },
     "require-dev": {
-        "phpunit/phpunit": "^6.5 || ^7.5 || ^8.5"
+        "phpunit/phpunit": "^5.5 || ^6.5 || ^7.5 || ^8.5"
     },
     "scripts": {
         "test": "phpunit",