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

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https://hg.mozilla.org/integration/gaia-central/rev/b3421d0cc5b3
Author: Alive.Kuo <alegnadise@gmail.com>
Desc: Merge pull request #17503 from alivedise/bugzilla/970028_master_v2/nicer-activity-animation

Bug 970028 - Nicer activity opener animation

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https://hg.mozilla.org/integration/gaia-central/rev/673fc5bf232d
Author: Alive Kuo <alegnadise@gmail.com>
Desc: Bug 970028 - Nicer activity opener animation
Bumping manifests a=b2g-bump