Change django.db.models.deletion.CASCADE to django.db.models.deletion.PROTECT while creating migrations

[Saurabh Agrawal]

By default, on deletion, the changes shouldn't be cascaded, but protected. Results in loss of data if the migrations generated are not checked carefully.

[Tim Graham]

Thanks for the suggestion, but it's obsolete because Django 1.9 ​deprecates the default value for on_delete so that in Django 2.0, the developer must specify a value.