composer: upgrade rector

Author: jtojnar

composer.json

@@ -35,7 +35,7 @@
         "symfony/phpunit-bridge": "^4.4|^5.3|^6.0",
         "phpstan/phpstan": "^1.3",
         "phpstan/phpstan-phpunit": "^1.0",
-        "rector/rector": "^0.12.15"
+        "rector/rector": "^0.15.0"
     },
     "suggest": {
         "ext-tidy": "Used to clean up given HTML and to avoid problems with bad HTML structure."