refactor and make gem rake task inclusion explicit

fixes: CNVS-12513

test plan:
- run rake tasks for i18n and jst

Change-Id: Ied2b1d67979e55356adfef1cc7aaed42aa0c4b5a
Reviewed-on: https://gerrit.instructure.com/33501
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>
QA-Review: Simon Williams <simon@instructure.com>
Reviewed-by: Jon Jensen <jon@instructure.com>