Foreign keys break on migration if models are renamed in a different app

[Christopher Neugebauer]

Steps to reproduce:

1. Create app a, with a model called A. Run makemigrations a
2. Create app b, with a model called B. Give B a ForeignKey to A. Run makemigrations b
3. Rename model A to Aa. Run makemigrations a
4. Run migrate

Expected behaviour:

The migration completes successfully

Actual behaviour:

Migration 0001 on b is run after migration 0002 on a, and the migration fails with the following error:

` raise ValueError('Related model %r cannot be resolved' % self.remote_field.model)
ValueError: Related model u'a.A' cannot be resolved`