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

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https://hg.mozilla.org/integration/gaia-central/rev/be996b54a6b9
Author: Timothy Guan-tin Chien <timdream@gmail.com>
Desc: Merge pull request #11934 from cvan/mkt-icons

Bug 901134 - Add higher-res Marketplace icon, r=timdream

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https://hg.mozilla.org/integration/gaia-central/rev/31d1637b109b
Author: Chris Van <cvan@mozilla.com>
Desc: add higher-res Marketplace icon (bug 901134)