Enhancement: Require phpstan/phpstan

Author: localheinz

composer.json

@@ -53,6 +53,7 @@
         "localheinz/test-util": "0.6.0",
         "mikey179/vfsStream": "^1.6.5",
         "mockery/mockery": "^1.0.0",
+        "phpstan/phpstan": "~0.9.2",
         "phpunit/phpunit": "^6.5.1",
         "symfony/browser-kit": "^3.4",
         "symfony/debug-bundle": "^3.4",