spec: xsslint bump, fixes CNVS-24917
spec: xsslint bump, fixes CNVS-24917

this pins the acorn version, so we have less strict `undefined` handling

test plan:
n/a, xsslint should pass again

Change-Id: I70ac1de7ce303ac23c70242e13933dbd5ab0c9d0
Reviewed-on: https://gerrit.instructure.com/66760
Reviewed-by: Clay Diffrient <9dff2e5c98626d20d2848250c411e8009465bb87@instructure.com>
Product-Review: Jon Jensen <44f878afe53efc66b76772bd845eb65944ed8232@instructure.com>
QA-Review: Jon Jensen <44f878afe53efc66b76772bd845eb65944ed8232@instructure.com>
Tested-by: Jenkins

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