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

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https://hg.mozilla.org/integration/gaia-central/rev/b71a21eeff88
Author: Yura Zenevich <yura.zenevich@gmail.com>
Desc: Merge pull request #19561 from yzen/bug-1014761

Bug 1014761 - improved the way utility tray visibility works with the sc...

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https://hg.mozilla.org/integration/gaia-central/rev/7166d2cf212a
Author: Yura Zenevich <yzenevich@mozilla.com>
Desc: Bug 1014761 - improved the way utility tray visibility works with the screen reader.
Bumping manifests a=b2g-bump