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

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https://hg.mozilla.org/integration/gaia-central/rev/6a8eef10964c
Author: Alive.Kuo <alegnadise@gmail.com>
Desc: Merge pull request #12782 from alivedise/bugzilla/925609/remove-windowmanager-screenshots

Bug 925609 - Remove WindowManager.screenshots, r=timdream

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https://hg.mozilla.org/integration/gaia-central/rev/72dd2d979c17
Author: Alive Kuo <alegnadise@gmail.com>
Desc: Bug 925609 - Remove WindowManager screenshots reference