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

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https://hg.mozilla.org/integration/gaia-central/rev/b3437e4a217b
Author: Cristian Rodriguez <crdlc@tid.es>
Desc: Merge pull request #20747 from crdlc/bug-1026091

Bug 1026091 - [Homescreen] 2.0 Smart Collections header style is not con...

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https://hg.mozilla.org/integration/gaia-central/rev/6f563b49429e
Author: crdlc <crdlc@tid.es>
Desc: Bug 1026091 - [Homescreen] 2.0 Smart Collections header style is not consistent with OS headers
Bumping manifests a=b2g-bump