Bumping gaia.json for 3 gaia-central revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/7864248beea9
Author: evelynhung <jj.evelyn@gmail.com>
Desc: Merge pull request #13129 from MBRSL/persona-fix

Bug 913385 - Hardware testing app, r=evelyn

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https://hg.mozilla.org/integration/gaia-central/rev/fa740ee56c00
Author: Tom Jao <tjao@mozilla.com>
Desc: Bug 913385 - Hardware testing app

Fix persona testing

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https://hg.mozilla.org/integration/gaia-central/rev/cfa8930a9f9f
Author: Timothy Guan-tin Chien <timdream@gmail.com>
Desc: Revert "Revert "Merge pull request #13115 from RudyLu/keyboard/Bug928281-keyboard_OOP""

This reverts commit 90fbd75eccd3408ca04907fd80b72275771b3b30.