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

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https://hg.mozilla.org/integration/gaia-central/rev/837c8251a901
Author: vingtetun <21@vingtetun.org>
Desc: Merge pull request #13996 from vingtetun/bug916185

Bug 916185 - Various gaia apps don't set a meta-viewport tag. r=gal

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https://hg.mozilla.org/integration/gaia-central/rev/39ba9c8034d9
Author: Vivien Nicolas <21@vingtetun.org>
Desc: Bug 916185 - Various gaia apps don't set a meta-viewport tag. r=gal