update ims-lti gem to fix specs

fixes PLAT-676

test-plan:
specs should pass

Change-Id: I39118e706ab6b6b8ed83f4914407ac2388cfb886
Reviewed-on: https://gerrit.instructure.com/42521
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brad Humphrey <brad@instructure.com>
Product-Review: Nathan Mills <nathanm@instructure.com>
QA-Review: Nathan Mills <nathanm@instructure.com>