Bumping gaia.json for 2 gaia-central revision(s)

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https://hg.mozilla.org/integration/gaia-central/rev/d4cff652d64a
Author: Albert <acperez@tid.es>
Desc: Merge pull request #11478 from acperez/bug-904004

Bug 904004 - Manage remote custom single variant apps at build time. r = yurenju

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https://hg.mozilla.org/integration/gaia-central/rev/f2c2e87a95a0
Author: Albert <acperez@tid.es>
Desc: Bug 904004 - Manage remote custom single variant apps at build time. r=yurenju