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

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https://hg.mozilla.org/integration/gaia-central/rev/9b1e1641e89c
Author: steveck-chung <schung@mozilla.com>
Desc: Merge pull request #10827 from steveck-chung/hdpi-image-assets-css

Bug 884975 - [MMS] Apply 1.5x Res Version of the Images. r=julienw

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https://hg.mozilla.org/integration/gaia-central/rev/4bd08888ab27
Author: Steve Chung <schung@mozilla.com>
Desc: Bug 884975 - [MMS] Apply 1.5x Res Version of the Images