Update phpstan/phpstan requirement from 0.12.3 to 0.12.4

Updates the requirements on [phpstan/phpstan](https://github.com/phpstan/phpstan) to permit the latest version.
- [Release notes](https://github.com/phpstan/phpstan/releases)
- [Commits](phpstan/phpstan@0.12.3...0.12.4)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

composer.json

@@ -29,7 +29,7 @@
     },
     "require-dev": {
         "phpunit/phpunit": "^7.0",
-        "phpstan/phpstan": "0.12.3",
+        "phpstan/phpstan": "0.12.4",
         "friendsofphp/php-cs-fixer": "2.16.1",
         "maglnet/composer-require-checker": "^1.1.0 || ^2.0.0",
         "phpro/grumphp": "^0.16.0"