Bumping gaia.json for 1 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/b21ee471b8fc
Author: Dale Harvey <dale@arandomurl.com>
Desc: Bug 1074909 - Only display loaded favicons. r=kgrandon
Bumping manifests a=b2g-bump