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

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https://hg.mozilla.org/integration/gaia-central/rev/676b3b7d9961
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Merge pull request #20929 from crdlc/bug-1029479

Bug 1029479 - [Vertical Homescreen] Edit header is not centered

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https://hg.mozilla.org/integration/gaia-central/rev/f56bbd7d5be6
Author: crdlc <crdlc@tid.es>
Desc: Bug 1029479 - [Vertical Homescreen] Edit header is not centered (smart collection does it properly)
Bumping manifests a=b2g-bump