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

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https://hg.mozilla.org/integration/gaia-central/rev/63b137e2b986
Author: vingtetun <21@vingtetun.org>
Desc: Merge pull request #15995 from vingtetun/bug967878

Bug 967878 - [Messages] Background doesn't scroll with the threads causi...

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https://hg.mozilla.org/integration/gaia-central/rev/3282fcb707c5
Author: Vivien Nicolas <21@vingtetun.org>
Desc: Bug 967878 - [Messages] Background doesn't scroll with the threads causing expensive over-invalidation. r=felash,bgirard
Bumping manifests a=b2g-bump