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

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https://hg.mozilla.org/integration/gaia-central/rev/a79efee39d18
Author: Cristian Rodriguez <crdlc@tid.es>
Desc: Merge pull request #13439 from eeejay/bug-930587

Bug 930587 - [homescreen] Use aria-hidden on offscreen pages instead of ...

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https://hg.mozilla.org/integration/gaia-central/rev/84fadf5552b4
Author: Eitan Isaacson <eitan@monotonous.org>
Desc: Bug 930587 - [homescreen] Use aria-hidden on offscreen pages instead of visibility for perf.