Update phpstan/phpstan requirement from ^0.12 to ^1.4

Updates the requirements on [phpstan/phpstan](https://github.com/phpstan/phpstan) to permit the latest version.
- [Release notes](https://github.com/phpstan/phpstan/releases)
- [Changelog](https://github.com/phpstan/phpstan/blob/master/CHANGELOG.md)
- [Commits](phpstan/phpstan@0.12.0...1.4.0)

---
updated-dependencies:
- dependency-name: phpstan/phpstan
  dependency-type: direct:development
...

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

composer.json

@@ -70,7 +70,7 @@
     "friendsofphp/php-cs-fixer": "^2.8.1",
     "prooph/php-cs-fixer-config": "^0.4",
     "matthiasnoback/symfony-dependency-injection-test": "^3.1 || ^4.1",
-    "phpstan/phpstan": "^0.12"
+    "phpstan/phpstan": "^1.4"
   },
   "minimum-stability": "dev",
   "prefer-stable": true,