fix config deprecation warnings in Rails 4

Change-Id: I7f42ef939d2a707c5df657aceef59ee7ace5071b
Reviewed-on: https://gerrit.instructure.com/41576
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>