fix double requiring of application.rb

This happens when script/rails is executed via a symlinked canvas
directory

Change-Id: I64fc408244ead13c3c78580c3b3e72102445df34
Reviewed-on: https://gerrit.instructure.com/40740
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>