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

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https://hg.mozilla.org/integration/gaia-central/rev/19ff5c952f8b
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Bug 1019841 - [Gallery] Linter follow-up, update to use <gaia-header> r=kgrandon
Bumping manifests a=b2g-bump