add dummy Scribd::Document

fixes CNVS-16347

this fixes version deserialization when a version references a scribd
doc

Change-Id: Id29b6ed9080935b35aa1c5905ddea93e330a43a1
Reviewed-on: https://gerrit.instructure.com/43115
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Josh Simpson <jsimpson@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>
QA-Review: Cameron Matheson <cameron@instructure.com>