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

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https://hg.mozilla.org/integration/gaia-central/rev/7c8b471791f7
Author: Yuren Ju <yurenju@gmail.com>
Desc: Merge pull request #16502 from vijayanandh/webappwarn

Bug 971648 - Add warning message if there is no r=@yurenju

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https://hg.mozilla.org/integration/gaia-central/rev/e19cd01233a2
Author: vijayanandh <vijayanand.hongal@accenture.com>
Desc: Bug 971648 - Add warning message if there is no
 manifest.webapp or update.webapp in an app directory
Bumping manifests a=b2g-bump