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

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https://hg.mozilla.org/integration/gaia-central/rev/769491b13858
Author: Chris Lord <chrislord.net@gmail.com>
Desc: Merge pull request #26415 from Cwiiis/bug1098480-landscape-system-update

Bug 1098480 - Fix system update splash in landscape. r=alive

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https://hg.mozilla.org/integration/gaia-central/rev/59a1496e986d
Author: Chris Lord <chrislord.net@gmail.com>
Desc: Bug 1098480 - Fix system update splash in landscape. r=alive
Bumping manifests a=b2g-bump