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

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https://hg.mozilla.org/integration/gaia-central/rev/c8c855f84668
Author: Amir Nissim <amirnissim@gmail.com>
Desc: Merge pull request #18123 from EverythingMe/960720-collection-bg

Bug 960720 - Change the client side Smart Collection wallpapers

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https://hg.mozilla.org/integration/gaia-central/rev/bf21a848a0d8
Author: Amir Nissim <amirnissim@gmail.com>
Desc: Bug 960720 - Change the client side Smart Collection wallpapers [r=ranbena]
Bumping manifests a=b2g-bump