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

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https://hg.mozilla.org/integration/gaia-central/rev/904f39efec16
Author: Timothy Guan-tin Chien <timdream@gmail.com>
Desc: Merge pull request #11065 from rexboy7/fixhd-bug895353-master

Bug 895353 - [wallpaper] hdpi/xhdpi resolution assets and layout for Wallpaper app, r=timdream

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https://hg.mozilla.org/integration/gaia-central/rev/c3afdcf4f6d5
Author: Rex Lee <rexboy@mozilla.com>
Desc: Bug 895353 - [wallpaper] hdpi/xhdpi resolution assets and layout for Wallpaper app