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

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https://hg.mozilla.org/integration/gaia-central/rev/b47135410721
Author: Borja Salguero <borjasalguero@users.noreply.github.com>
Desc: Merge pull request #14101 from borjasalguero/fxa_module

Bug 929388 - Incorporate a FxA Manager and FxA Client into the System app r=alive, ferjm, arcturus

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https://hg.mozilla.org/integration/gaia-central/rev/18b7c58f67b6
Author: Borja Salguero <borja.salguero@gmail.com>
Desc: Bug 929388 - Incorporate a FxA Manager and FxA Client into the System app r=alive, ferjm, arcturus