fix canvas_stringex gem spec for rails 3

Change-Id: Ibb5a61130a0d0cf9b2b5d97eaa06d8b30711f9bf
Reviewed-on: https://gerrit.instructure.com/29856
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>