Download the WordPress GCS plugin from wordpress.org (GoogleCloudPlatform#326)

* Download the WordPress GCS plugin from wordpress.org

* Use WordPress gcs plugin 0.1.2

Author: Takashi Matsuo

appengine/wordpress/src/WordPressSetup.php

@@ -37,7 +37,7 @@ class WordPressSetup extends Command
     const LATEST_GAE_WP =
         'https://downloads.wordpress.org/plugin/google-app-engine.1.6.zip';
     const LATEST_GCS_PLUGIN =
-        'https://github.com/GoogleCloudPlatform/wordpress-plugins/releases/download/gcs-0.1/gcs-0.1.zip';
+        'https://downloads.wordpress.org/plugin/gcs.0.1.2.zip';
 
     const FLEXIBLE_ENV = 'Flexible Environment';
     const STANDARD_ENV = 'Standard Environment';