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

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https://hg.mozilla.org/integration/gaia-2_0/rev/1639d8ef31ab
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Bug 1050339 - [B2G][Smart Collections] Default Smart Collections are missing their icons r=yurenju
Bumping manifests a=b2g-bump