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

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https://hg.mozilla.org/integration/gaia-central/rev/3b76779650c6
Author: autolander <bug.autolander@gmail.com>
Desc: Bug 1102967 - merge pull request #26597 from KevinGrandon:bug_1102967_give_me_button to mozilla-b2g:master

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https://hg.mozilla.org/integration/gaia-central/rev/7ce5fa9bee65
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Bug 1102967 - Cram unread button in message UI
Bumping manifests a=b2g-bump