fix duplicate dependency in event_stream spec

to avoid bundler warnings

Change-Id: Ie997a7bb06c6d3334ea8973a80902a5239f67eb5
Reviewed-on: https://gerrit.instructure.com/35516
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>