fix thumbnail processing in ruby 2

Change-Id: I3826fe581d674e1f55b05823c8be3a6a4cee03ec
Reviewed-on: https://gerrit.instructure.com/36847
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>