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

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https://hg.mozilla.org/integration/gaia-central/rev/7831dcded4df
Author: Fabrice Desré <fabrice@desre.org>
Desc: Merge pull request #15640 from fabricedesre/no-compress

Bug 962976 - Use a blacklist to avoid compressing specific file types wh...

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https://hg.mozilla.org/integration/gaia-central/rev/2f5baa0381a4
Author: Fabrice Desre <fabrice@desre.org>
Desc: Bug 962976 - Use a blacklist to avoid compressing specific file types which are already compressed r=yurenju
Bumping manifests a=b2g-bump