create temporary records with .scoped.new for rails 3 compatibility

in rails 3, .new is analogous to .build for collections, so otherwise
temporary records will be added to the collection (and be autosaved)

Change-Id: I08ce7b4b35ee35646aff3afd237bbe896162a014
Reviewed-on: https://gerrit.instructure.com/30334
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>