Set a version range for the django-multiselectfield dependency.
Set a version range for the django-multiselectfield dependency.

The `django-multiselectfield` dependency didn't have a range specified.
We had a user who had an old version installed, which caused some
crashes. We now set a required range, ensuring this doesn't happen
in future versions of Review Board.

Testing Done:
Successfully installed a package with the dependency.

Reviewed at https://reviews.reviewboard.org/r/11682/

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