add missing dropped columns to dropped columns.rb

refs CNVS-15481

Change-Id: Id9e0a0ce5fc671b52948a1376e95bf54b57c471a
Reviewed-on: https://gerrit.instructure.com/44256
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>
QA-Review: Rob Orton <rob@instructure.com>