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

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https://hg.mozilla.org/integration/gaia-central/rev/8dc29686ea31
Author: Wilson Page <wilsonpage@me.com>
Desc: Merge pull request #19693 from yor-mozilla-com/1011807

Bug 1011807 - Handle attribute change on gaia-header

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https://hg.mozilla.org/integration/gaia-central/rev/47f1d0946fde
Author: Yan Or <yor@mozilla.com>
Desc: Bug 1011807 - Handle attribute change on gaia-header
Bumping manifests a=b2g-bump