fix selenium after hooks for rails 3

replace append_after with after so the ajax requests are
completed before the teardown_fixtures hook is run

Change-Id: I2ae3d5470bf6a0b5b6ab6574e818eb1df6d416d5
Reviewed-on: https://gerrit.instructure.com/30293
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>