Support for Django 1.7 breaks avocado metadata migrations

[tjrivera]

The move to Django 1.7 moves us away from south to Django's native migration platform. As a result, the avocado migration management subcommand will have to be updated as it relies directly on south to facilitate migrations.

This might be an opportunity to explore more fine grained management or "merges" of metadata from one serialized migration to another depending on the goals of the Harvest road map.