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

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https://hg.mozilla.org/integration/gaia-central/rev/756943e4bf17
Author: Cristian Rodriguez <crdlc@tid.es>
Desc: Merge pull request #20216 from crdlc/bug-1020134-revert

Bug 1020134 revert

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https://hg.mozilla.org/integration/gaia-central/rev/b692c7ba4ea0
Author: crdlc <crdlc@tid.es>
Desc: Revert "Bug 1020134 - Add backgrounds to pre-defined collectionsBug 1020134 - Add backgrounds to pre-defined collections"

This reverts commit 054e1c6852a0fefcff08448792cf987e9aef9c6e.
Bumping manifests a=b2g-bump