fix canvas_stringex dependency conflict for rails 3

Change-Id: Id9eb79fb2a4ea546eded22cc4d92e35e2162a7db
Reviewed-on: https://gerrit.instructure.com/29514
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>