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

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https://hg.mozilla.org/integration/gaia-central/rev/28244f65faf5
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Merge pull request #21703 from KevinGrandon/bug_1038178_vertical_scrollroot

Bug 1038178 - [Vertical Homescreen] Use scrollbar from root scrollframe

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https://hg.mozilla.org/integration/gaia-central/rev/db122d234a27
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Bug 1038178 - [Vertical Homescreen] Use scrollbar from root scrollframe r=Cwiiis
Bumping manifests a=b2g-bump