bump multi_json to 1.15.0
bump multi_json to 1.15.0

https://github.com/intridea/multi_json/blob/9cc87105d12f72c85d618ca65a214fc49d027462/CHANGELOG.md

refs FOO-1219
flag = none
test plan: specs pass

Change-Id: I238bae5a7d76aed42e607279243beae952ff56b7
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/253004
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: August Thornton <9adc7a1161ddf32ff608de792a7e50179545f026@instructure.com>
QA-Review: August Thornton <9adc7a1161ddf32ff608de792a7e50179545f026@instructure.com>
Product-Review: August Thornton <9adc7a1161ddf32ff608de792a7e50179545f026@instructure.com>

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