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

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https://hg.mozilla.org/integration/gaia-central/rev/15fe9d38266b
Author: gasolin <gasolin@gmail.com>
Desc: Merge pull request #13684 from gasolin/issue-936395

Bug 936395 - [Notification] Notification counter doesn't work correctly, r=alive

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https://hg.mozilla.org/integration/gaia-central/rev/cc1c7a3aba63
Author: gasolin <gasolin@gmail.com>
Desc: Bug 936395 - [Notification] Notification counter doesn't work correctly