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

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https://hg.mozilla.org/integration/gaia-central/rev/6ff180a83c70
Author: Aleh Zasypkin <azasypkin@mozilla.com>
Desc: Bug 951687 - [Messages] Thread. Update file icon placeholders r=julien
Bumping manifests a=b2g-bump