replace Array#once_per extension with #uniq

Change-Id: I49111316d6cbc1f4be111567ed7f5e670d1d948c
Reviewed-on: https://gerrit.instructure.com/30023
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Zach Pendleton <zachp@instructure.com>
Product-Review: Zach Pendleton <zachp@instructure.com>
QA-Review: Stephan Hagemann <stephan@pivotallabs.com>