fix dropped_columns for rails4

rails4 takes a second arg, we don't use it, just pass it on

Change-Id: Ice2ab179521a8aa85994ac6cce8fdd140a80628b
Reviewed-on: https://gerrit.instructure.com/41063
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Nick Cloward <ncloward@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>
QA-Review: Brian Palmer <brianp@instructure.com>