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

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https://hg.mozilla.org/integration/gaia-central/rev/3d294ffa51af
Author: Etienne Segonzac <etienne@segonzac.info>
Desc: Merge pull request #16715 from etiennesegonzac/bug-969315-final-app-to-app-transition

Bug 969315 - Implement the final 1.4 version of the app to app swipe transition r=alive

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https://hg.mozilla.org/integration/gaia-central/rev/4d8c99aa8801
Author: Etienne Segonzac <etienne@segonzac.info>
Desc: Bug 969315 - Implement the final 1.4 version of the app to app swipe transition.
Bumping manifests a=b2g-bump