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

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https://hg.mozilla.org/integration/gaia-central/rev/481bc5b521cd
Author: steveck-chung <schung@mozilla.com>
Desc: Merge pull request #11209 from steveck-chung/hdpi-image-assets

Bug 898299 - hdpi/xhdpi Image assets polishing in Message app. r=rexboy

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https://hg.mozilla.org/integration/gaia-central/rev/680657d97a21
Author: Steve Chung <schung@mozilla.com>
Desc: Bug 898299 - hdpi/xhdpi Image assets polishing in Message app