[Fixed]-Move models between Django (1.8) apps with ForeignKey references

I found the error, I left this information in case anybody has the same problem.

The problem was really induced by the rename of old_app.Car to old_app.NewCar.

makemigrations detected the change and created two migrations one in old_app with the rename and other on new_app updating the ForeingKey. The problem was that you have to manually add in new_app dependencies the old_app migration where the model was renamed.