Bumping gaia.json for 2 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/df8bbd05f390
Author: Marcus Cavanaugh <m@mcav.com>
Desc: Merge pull request #17915 from mcav/alarm-volume-3

Bug 939197 - [Clock] Alarms need volume control. r=millermedeiros

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https://hg.mozilla.org/integration/gaia-central/rev/690b730a0f3a
Author: Marcus Cavanaugh <m@mcav.com>
Desc: Bug 939197 - [Clock] Alarms need volume control. r=millermedeiros
Bumping manifests a=b2g-bump