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

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https://hg.mozilla.org/integration/gaia-central/rev/7a0a8189617b
Author: vingtetun <21@vingtetun.org>
Desc: Merge pull request #17128 from vingtetun/bug977680

Bug 977680 - [Messages] Remove position: sticky as it hurts scrolling pe...

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https://hg.mozilla.org/integration/gaia-central/rev/007b0102dc88
Author: Vivien Nicolas <21@vingtetun.org>
Desc: Bug 977680 - [Messages] Remove position: sticky as it hurts scrolling performance. r=kgrandon,f=felash
Bumping manifests a=b2g-bump