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

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https://hg.mozilla.org/integration/gaia-central/rev/8547b0e2b40e
Author: gasolin <gasolin@gmail.com>
Desc: Merge pull request #13582 from gasolin/issue-936997

Bug 936997 - [Flatfish][Window Mgmt] Make modal dialogs flexible, r=alive

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https://hg.mozilla.org/integration/gaia-central/rev/31f6dbd87a65
Author: gasolin <gasolin@gmail.com>
Desc: Bug 936997 - [Flatfish][Window Mgmt] Make attention dialogs flexible