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

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https://hg.mozilla.org/integration/gaia-central/rev/18c3b948728e
Author: Pavel Ivanov <pivanov@mozilla.com>
Desc: Merge pull request #22914 from pivanov/bug-1035680

Bug 1035680 - [FTU] - check missing images

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https://hg.mozilla.org/integration/gaia-central/rev/fb881bfc2318
Author: Pavel Ivanov <pivanov@mozilla.com>
Desc: Bug 1035680 - [FTU] - check missing images
Bumping manifests a=b2g-bump