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

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https://hg.mozilla.org/integration/gaia-central/rev/a0ffe2f47707
Author: Pavel Ivanov <pivanov@mozilla.com>
Desc: Merge pull request #17023 from pivanov/bug-968483

Bug 968483 - [Building Blocks] Update header design to accommodate larger back-button

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https://hg.mozilla.org/integration/gaia-central/rev/efaaabc5b3b4
Author: Pavel Ivanov <pivanov@mozilla.com>
Desc: Bug 968483 - [Building Blocks] Update header design to accommodate larger back-button
Bumping manifests a=b2g-bump