Fixed the way that the CustomInlineFormset class gets used.

Rather than executing inlineformset_factory() in the constructor,
SortableInlineAdminMixin now specifies the 'formset' class attribute just
like admin.InlineModelAdmin does it. Since users already have to put the
SortableInlineAdminMixin before admin.TabularInline in the class definition,
this means that admin.InlineModelAdmin.formset gets overridden, letting
admin.InlineModelAdmin.get_formset() do it's job as usual.