inline only usage of Array#clump_per

Change-Id: Iaf4c85a6c89e693010e2da3c5830ef496a81ef32
Reviewed-on: https://gerrit.instructure.com/30024
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Raphael Weiner <rweiner@pivotallabs.com>
Product-Review: Raphael Weiner <rweiner@pivotallabs.com>
QA-Review: Raphael Weiner <rweiner@pivotallabs.com>