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

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https://hg.mozilla.org/integration/gaia-central/rev/781ee1f0b6da
Author: Hubert Figuière <hub@figuiere.net>
Desc: Merge pull request #21450 from hfiguiere/bug1027232-2

Bug 1027232 - Wait for the homescreen to be ready

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https://hg.mozilla.org/integration/gaia-central/rev/cb3f3e820a41
Author: Hubert Figuière <hub@figuiere.net>
Desc: Bug 1027232 - Wait for the homescreen to be ready
Bumping manifests a=b2g-bump