Fix issue #274 (#275)
Fix issue #274 (#275)

Use `set` method instead of `__set__` on a
`ManyToManyField` to ensure compatibility with
Django 2.0 and higher.
mtford90/silk,django-silk/silk,django-silk/silk,django-silk/silk,jazzband/silk,jazzband/silk,crunchr/silk,crunchr/silk,jazzband/silk,mtford90/silk,crunchr/silk,django-silk/silk,mtford90/silk,jazzband/silk,crunchr/silk,mtford90/silk