clean up stringex gem tests

* clean up sqlite db after tests run in stringex gem
* test Redcloth correctly in stringex gem
* move database cleanup to test runner in stringex
* keep rails2 Gemfile.lock after running gem test runners

closes CNVS-11020

test plan:
- stringex tests should still pass in rails 2.3 and 3

Change-Id: Idde773ddf3c8158bb0040a2eae9d4ecc8c8c2d15
Reviewed-on: https://gerrit.instructure.com/29781
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Liz Abinante <labinante@instructure.com>
Product-Review: Bryan Madsen <bryan@instructure.com>
QA-Review: Bryan Madsen <bryan@instructure.com>
Reviewed-by: Raphael Weiner <rweiner@pivotallabs.com>
Reviewed-by: Stephan Hagemann <stephan@pivotallabs.com>