Fixing AlterSortedManyToManyField operation for postgres databases.
Fixing AlterSortedManyToManyField operation for postgres databases.

fabrique/django-sortedm2m,gregmuellegger/django-sortedm2m,gradel/django-sortedm2m,fabrique/django-sortedm2m,gregmuellegger/django-sortedm2m,gradel/django-sortedm2m,gregmuellegger/django-sortedm2m,gradel/django-sortedm2m,fabrique/django-sortedm2m