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

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https://hg.mozilla.org/integration/gaia-central/rev/4b5986581fb2
Author: Eitan Isaacson <eitan@monotonous.org>
Desc: Merge pull request #16051 from yzen/957674

Bug 957674 - adding an easy way to enable/disable a screen reader. r=alive

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https://hg.mozilla.org/integration/gaia-central/rev/8f45be5883eb
Author: Yura Zenevich <yzenevich@mozilla.com>
Desc: Bug 957674 - adding an easy way to enable/disable a screen reader.
Bumping manifests a=b2g-bump