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

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https://hg.mozilla.org/integration/gaia-central/rev/e88fea50cbf4
Author: Mason Chang <mason@masonchang.com>
Desc: Merge pull request #16647 from changm/bug975811

Bug 975811. Background position of homescreen corrected. r=alive

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https://hg.mozilla.org/integration/gaia-central/rev/796d7ff5022b
Author: Mason Chang <mchang@mozilla.com>
Desc: Bug 975811. Background position of homescreen corrected
Bumping manifests a=b2g-bump