fix app_api for ruby 2.1

it expects a string, not an array (and always has; just stricter now)

Change-Id: I704610179efec62a9f5ebc8a08444ac7aa5e9790
Reviewed-on: https://gerrit.instructure.com/36745
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Nick Cloward <ncloward@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>