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

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https://hg.mozilla.org/integration/gaia-central/rev/75ba63260558
Author: Yura Zenevich <yura.zenevich@gmail.com>
Desc: Merge pull request #19825 from yzen/bug-1014824

Bug 1014824 - improving FTU app accessibility around screen/dialog visib...

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https://hg.mozilla.org/integration/gaia-central/rev/b853afc650cf
Author: Yura Zenevich <yzenevich@mozilla.com>
Desc: Bug 1014824 - improving FTU app accessibility around screen/dialog visibility. r=fcampo
Bumping manifests a=b2g-bump