use syck for yaml parsing in canvas_stringex gem tests

it already inherits this parser from canvas, but when running the tests in
isolation it needs to be told to use it explicitly.

closes CNVS-11018

test plan:
- stringex tests should pass in rails 2.3

Change-Id: I280c01523e5f66024a225d2a9cc50ac79896de17
Reviewed-on: https://gerrit.instructure.com/29734
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>