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

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https://hg.mozilla.org/integration/gaia-central/rev/4d75d22d09b4
Author: Timothy Guan-tin Chien <timdream@gmail.com>
Desc: Merge pull request #15052 from cvan/mkt-icons

Bug 944224 - Forgo data URIs for Marketplace homescreen icons, r=fabrice

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https://hg.mozilla.org/integration/gaia-central/rev/04c230d0fb9b
Author: Christopher Van <cvan@mozilla.com>
Desc: Bug 944224 - Forgo data URIs for Marketplace homescreen icons