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

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https://hg.mozilla.org/integration/gaia-central/rev/d8ef3f4a989c
Author: vingtetun <21@vingtetun.org>
Desc: Merge pull request #17296 from vingtetun/bug978063

Bug 978063 - [BuildingBlocks] Favor scroll performance over thumbnails q...

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https://hg.mozilla.org/integration/gaia-central/rev/f6787e125b64
Author: Vivien Nicolas <21@vingtetun.org>
Desc: Bug 978063 - [BuildingBlocks] Favor scroll performance over thumbnails quality. r=arnau
Bumping manifests a=b2g-bump