Build(deps-dev): Bump phpstan/phpstan from 0.11 to 0.11.15

Bumps [phpstan/phpstan](https://github.com/phpstan/phpstan) from 0.11 to 0.11.15.
- [Release notes](https://github.com/phpstan/phpstan/releases)
- [Commits](phpstan/phpstan@0.11...0.11.15)

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

composer.json

@@ -29,7 +29,7 @@
     "localheinz/php-cs-fixer-config": "~1.22.1",
     "localheinz/phpstan-rules": "~0.7.1",
     "localheinz/test-util": "~0.7.0",
-    "phpstan/phpstan": "~0.11.0",
+    "phpstan/phpstan": "~0.11.15",
     "phpstan/phpstan-deprecation-rules": "~0.11.0",
     "phpstan/phpstan-strict-rules": "~0.11.0",
     "phpunit/phpunit": "^6.5.13 || ^7.5.2",