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

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https://hg.mozilla.org/integration/gaia-central/rev/c6a4b73e5347
Author: vingtetun <21@vingtetun.org>
Desc: Merge pull request #15989 from staktrace/nocheck-setting

Bug 967502 - Expose gaia developer setting for no checkerboarding. r=vin...

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https://hg.mozilla.org/integration/gaia-central/rev/e08cc546ae66
Author: Kartikaya Gupta <kgupta@mozilla.com>
Desc: Bug 967502 - Expose gaia developer setting for no checkerboarding. r=vingtetun
Bumping manifests a=b2g-bump