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

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https://hg.mozilla.org/integration/gaia-central/rev/d1729e9d8fcc
Author: Mason Chang <mason@masonchang.com>
Desc: Merge pull request #16640 from changm/bug970284

Bug 970284 - Put Background in both System and Homescreen App. r=alive

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https://hg.mozilla.org/integration/gaia-central/rev/588d2988ee75
Author: Mason Chang <mchang@mozilla.com>
Desc: Bug 970284 - Put Background in both System and Homescreen App
Bumping manifests a=b2g-bump