avoid using a relative path for gem plugin loading

Change-Id: I33108ffccaa26a5995543e740f2696d4829254c4
Reviewed-on: https://gerrit.instructure.com/41358
Reviewed-by: Dave Donahue <ddonahue@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>