Fix initial Django 1.7 migration, with new model format change.

This reflects the model change in 5db650d2c6a6d2dd1004cb0a4a902a39ba75fbc6
The Django 1.7 version is intended to be used after migrating everything
on Django 1.6 first (there are other migrations between both releases after all).
Hence, there is no 0002 migration but rather an improvement of the initial migration.