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

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https://hg.mozilla.org/integration/gaia-central/rev/83a5223a226a
Author: Fabrice Desré <fabrice@desre.org>
Desc: Merge pull request #15341 from fabricedesre/mem-pressure-kbd

Bug 957880 - Expose memory pressure events to gaia's system app r=timdre...

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https://hg.mozilla.org/integration/gaia-central/rev/835c0bd17799
Author: Fabrice Desre <fabrice@desre.org>
Desc: Bug 957880 - Expose memory pressure events to gaia's system app r=timdream