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

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https://hg.mozilla.org/integration/gaia-central/rev/31a029b77c72
Author: Pavel Ivanov <pivanov@mozilla.com>
Desc: Merge pull request #14543 from pivanov/bug-930600

Bug 930600 - Add new Illustrative Wallpapers for v.1.3

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https://hg.mozilla.org/integration/gaia-central/rev/70d3a0ffa329
Author: Pavel Ivanov <pivanov@mozilla.com>
Desc: Bug 930600 - Add new Illustrative Wallpapers for v.1.3