cleanup stringex Gemfile slightly

Change-Id: I983d64ab39ccfe8af8c48100e0074400c43d0443
Reviewed-on: https://gerrit.instructure.com/30766
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>