Bumping gaia.json for 1 gaia-central revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/df31e3d14be6
Author: Zac Campbell <zcampbell@mozilla.com>
Desc: Revert "Bug 921721 - hotfix to bust JS Exception in startup a=testonly r=me"

This reverts commit 5cf27888e390526c072d4b44a5af54c8aaf7205a.