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

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https://hg.mozilla.org/integration/gaia-central/rev/2cbab6f51d9a
Author: George <georgeiscoming@gmail.com>
Desc: Merge pull request #10775 from cctuan/master-hd

Bug 881148 - hdpi/xhdpi resolution assets and layout for Gallery app

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https://hg.mozilla.org/integration/gaia-central/rev/302b70caf57d
Author: gduan <gduan@mozilla.com>
Desc: Bug 881148 - hdpi/xhdpi resolution assets and layout for Gallery app - update images