Update phpstan/phpstan requirement from 0.12.39 to 0.12.40 (#17)

Author: dependabot-preview[bot]

composer.json

@@ -8,7 +8,7 @@
         "php-64bit": "*"
     },
     "require-dev": {
-        "phpstan/phpstan": "0.12.39",
+        "phpstan/phpstan": "0.12.40",
         "phpstan/extension-installer": "^1.0",
         "phpstan/phpstan-strict-rules": "^0.12.4"
     },