[1.8.x] Fixed allow_migrate() signature in router examples.
[1.8.x] Fixed allow_migrate() signature in router examples.

Backport of 937643a1f215d7d2063b256269d757a2ee316f07 from master

h4r5h1t/django-hauthy,h4r5h1t/django-hauthy,h4r5h1t/django-hauthy,h4r5h1t/django-hauthy,pquentin/django,pquentin/django,pquentin/django