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

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https://hg.mozilla.org/integration/gaia-central/rev/503c38f7c1d9
Author: Fabien Cazenave <fabien@mozilla.com>
Desc: Merge pull request #16717 from fabi1cazenave/plural-bug977627

Bug 977627 - [Download Manager] Files delete notification lacks plural support

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https://hg.mozilla.org/integration/gaia-central/rev/8b785c064d4c
Author: Fabien Cazenave <fabien@cazenave.cc>
Desc: Bug 977627 - [Download Manager] Files delete notification lacks plural support, r=Pike
Bumping manifests a=b2g-bump