fix find_or_create_submissions

passing options to self.transaction (as opposed to
self.class.transaction) does not work in all versions
of rails

Change-Id: I438a1905f8aea44d87424390dd5c12bed950025a
Reviewed-on: https://gerrit.instructure.com/28513
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>