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

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https://hg.mozilla.org/integration/gaia-central/rev/c8ac536548db
Author: autolander <bug.autolander@gmail.com>
Desc: Bug 1103629 - merge pull request #26384 from KevinGrandon:bug_1103629_marionette_client_loader_verticalhome to mozilla-b2g:master

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https://hg.mozilla.org/integration/gaia-central/rev/a1796499b69e
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Bug 1103629 - [HomeScreen] Use a marionette client loader to include the home screen app
Bumping manifests a=b2g-bump