Add `on_delete` to ForeignKeys

[Epikem]

class Entry(models.Model):
    title = models.CharField(max_length=500)
    author = models.ForeignKey('auth.User')
    body = models.TextField()
    created_at = models.DateTimeField(auto_now_add=True, editable=False)
    modified_at = models.DateTimeField(auto_now=True, editable=False)

on_delete is required since django 1.9 and it raises type error

TypeError: __init__() missing 1 required positional argument: 'on_delete'