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

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https://hg.mozilla.org/integration/gaia-central/rev/e8cac2a23c22
Author: Fabien Cazenave <fabien@cazenave.cc>
Desc: Merge pull request #13972 from flodolo/bug942546

Bug 942546 - Use different labels for "Add to home screen" in buttons and headers, r=kaze

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https://hg.mozilla.org/integration/gaia-central/rev/7e967b387d8a
Author: Francesco Lodolo (:flod) <flod@lodolo.net>
Desc: Bug 942546 - Use different labels for "Add to home screen" in buttons and headers