bump react-router to 0.5.2

Change-Id: I5e097387cea46897f04d5005777d93e2cdd4cb68
Reviewed-on: https://gerrit.instructure.com/38918
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Clay Diffrient <cdiffrient@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>
QA-Review: Ryan Shaw <ryan@instructure.com>