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

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https://hg.mozilla.org/integration/gaia-central/rev/39d06bda5d40
Author: Carmen Jiménez <cjc@tid.es>
Desc: Merge pull request #20531 from mcjimenez/bug1024623

Bug 1024623 - [Vertical] Add collections to customization of vertical homescreen

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https://hg.mozilla.org/integration/gaia-central/rev/4fc040a41e9f
Author: Carmen Jimenez Cabezas <cjc@tid.es>
Desc: Bug 1024623 - [Vertical] Add collections to customization of vertical homescreen
Bumping manifests a=b2g-bump