pin the ims-lti gem

Change-Id: Ice371b54b6aa4781af2d7ef77201dfb201987cd2
Reviewed-on: https://gerrit.instructure.com/39421
Reviewed-by: Bryan Madsen <bryan@instructure.com>
Product-Review: Bryan Madsen <bryan@instructure.com>
QA-Review: Bryan Madsen <bryan@instructure.com>
Tested-by: Bryan Madsen <bryan@instructure.com>