add a dropped column to dropped_columns.rb

Change-Id: Ied363a8d60c917cdd5b1398e1ec69895dda4cdf9
Reviewed-on: https://gerrit.instructure.com/43105
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>