Allow wordpress-core-installer to write in composer.json (#367)

Author: danielbachhuber

composer.json

@@ -26,7 +26,8 @@
         "process-timeout": 7200,
         "sort-packages": true,
         "allow-plugins": {
-            "dealerdirect/phpcodesniffer-composer-installer": true
+            "dealerdirect/phpcodesniffer-composer-installer": true,
+            "johnpbloch/wordpress-core-installer": true
         }
     },
     "extra": {