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

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https://hg.mozilla.org/integration/gaia-central/rev/a4788658d9a4
Author: Yuren Ju <yurenju@gmail.com>
Desc: Merge pull request #16283 from yurenju/homescreen-makefile

Bug 968684 - Add Makefile for homescreen app r=@timdream

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https://hg.mozilla.org/integration/gaia-central/rev/c7bdf52d6617
Author: Yuren Ju <yurenju@gmail.com>
Desc: Bug 968684 - Add Makefile for homescreen app
Bumping manifests a=b2g-bump