Improve downgrade package calculation to be more robust
Improve downgrade package calculation to be more robust

ddrezga/CumulusCI,motivislearning/CumulusCI,SalesforceFoundation/CumulusCI,Joble/CumulusCI,steelbrick/CumulusCI,motivislearning/CumulusCI,steelbrick/CumulusCI,e02d96ec16/CumulusCI,cdcarter/CumulusCI,e02d96ec16/CumulusCI,roundcorner-products/CumulusCI,roundcorner-products/CumulusCI,SalesforceFoundation/CumulusCI,ddrezga/CumulusCI