Merge pull request #441 from wp-cli/update/allow-wordpress-core-installer

Allow `johnpbloch/wordpress-core-installer` to write

Author: danielbachhuber

composer.json

@@ -46,7 +46,8 @@
     },
     "config": {
         "allow-plugins": {
-            "dealerdirect/phpcodesniffer-composer-installer": true
+            "dealerdirect/phpcodesniffer-composer-installer": true,
+            "johnpbloch/wordpress-core-installer": true
         },
         "platform": {
             "php": "5.6"