Fix Appveyor build

PHPunit 8.3 is not compatible with symfony/phpunit-bridge <=4.3.3

Author: Jeremiah VALERIE

composer.json

@@ -50,7 +50,7 @@
  },
  "require-dev": {
  "doctrine/orm": "^2.5",
- "phpunit/phpunit": "^7.2 || ^8.2",
+ "phpunit/phpunit": "<8.3",
  "react/promise": "^2.5",
  "symfony/asset": "^3.4 || ^4.0",
  "symfony/browser-kit": "^3.4 || ^4.0",