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

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https://hg.mozilla.org/integration/gaia-central/rev/d6b9d136b50d
Author: vingtetun <21@vingtetun.org>
Desc: Merge pull request #22457 from vingtetun/background-color-landing

Bug 1047829 - Set a background-color on the screenshot-overlay. r=kgrand...

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https://hg.mozilla.org/integration/gaia-central/rev/23b5d094d063
Author: Vivien Nicolas <vnicolas@mozilla.com>
Desc: Bug 1047829 - Set a background-color on the screenshot-overlay. r=kgrandon
Bumping manifests a=b2g-bump