Rollback phpunit to support php 5.6+

Author: Tucker-Eric

composer.json

@@ -14,7 +14,7 @@
  "illuminate/support": "~5.0"
  },
  "require-dev": {
- "phpunit/phpunit": "^7.2"
+ "phpunit/phpunit": "~5.4.0"
  },
  "license": "MIT",
  "authors": [