Fix invalid registries JSON generation for Getting Started Addon

Steps to Reproduce:

• In nos-test, set Target Platform to 10.2.
• Add Web UI, DAM and Getting started with the Nuxeo Platform packages.
  => Error message appears:

  Failed to load registries for package 'nuxeo-platform-getting-started-1.3.1'. Error message is: 'java.lang.ClassCastException: net.sf.json.JSONArray cannot be cast to net.sf.json.JSONObject'