rails 3 compatibility for stringex gem

closes CNVS-11019

test plan:
- stringex tests should pass in rails3

Change-Id: I2768723eb37cf77deafc4cf567cf314006e17c93
Reviewed-on: https://gerrit.instructure.com/29747
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>
QA-Review: Simon Williams <simon@instructure.com>