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

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https://hg.mozilla.org/integration/gaia-central/rev/dff8d6c8c318
Author: Etienne Segonzac <etienne@segonzac.info>
Desc: Merge pull request #20901 from etiennesegonzac/bug-1028002

Bug 1028002 - Make sure an appWindow is in 'opened' state once we got the animationend and mozbrowserloadened instead of waiting for the opening timeout. r=timdream

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https://hg.mozilla.org/integration/gaia-central/rev/89158a4efc82
Author: Etienne Segonzac <etienne@segonzac.info>
Desc: Bug 1028002 - Make sure an appWindow is in 'opened' state once we got the animationend and mozbrowserloadened instead of waiting for the opening timeout.
Bumping manifests a=b2g-bump