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

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https://hg.mozilla.org/integration/gaia-central/rev/31acef1532cc
Author: George <georgeiscoming@gmail.com>
Desc: Merge pull request #18308 from cctuan/968661-3

Bug 968661 - Extract new build module webapp-shared.js from webapp-zip.j...

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https://hg.mozilla.org/integration/gaia-central/rev/fb2f6723539a
Author: cctuan <georgeiscoming@gmail.com>
Desc: Bug 968661 - Extract new build module webapp-shared.js from webapp-zip.js
Bumping manifests a=b2g-bump