fix parallel sis batch progress interval

the marshalling to/from the child serialized delayed job isn't including
@last_progress_update, causing errors comparing a time to nil.

refs CNVS-14640

Change-Id: I6636335202921c0210edce2ebebd4eef4ec22dac
Reviewed-on: https://gerrit.instructure.com/39358
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>
QA-Review: Brian Palmer <brianp@instructure.com>