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

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https://hg.mozilla.org/integration/gaia-central/rev/20258a9a0b11
Author: Etienne Segonzac <etienne@segonzac.info>
Desc: Merge pull request #10929 from lissyx/bug892520

Bug 892520 - Ensure serialization of upgrade path

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https://hg.mozilla.org/integration/gaia-central/rev/bc8ab7a1d3bd
Author: Alexandre Lissy <lissyx+github@lissyx.dyndns.org>
Desc: Bug 892520 - Ensure proper upgrade path