Bumping gaia.json for 2 gaia-central revision(s)

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https://hg.mozilla.org/integration/gaia-central/rev/bb346d13970d
Author: George <georgeiscoming@gmail.com>
Desc: Merge pull request #11035 from cctuan/system-image

Bug 891807 - @1.5x assets are required for system app

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https://hg.mozilla.org/integration/gaia-central/rev/6fa7c6cbea1f
Author: cctuan <georgeiscoming@gmail.com>
Desc: Bug 891807 - @1.5x assets are required for system app