improve blackboard true/false question parsing

test plan:
* import the first package referenced in the ticket
 (with "BBtestmgr" in the title)
* should bring in the true/false questions without errors

closes #CNVS-14569

Change-Id: Ibe56af41852234c7e982fcb37166ad5c55e61fda
Reviewed-on: https://gerrit.instructure.com/38801
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Bracken Mosbacker <bracken@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>