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

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https://hg.mozilla.org/integration/gaia-central/rev/683f9932ab8f
Author: Cristian Rodriguez <crdlc@tid.es>
Desc: Merge pull request #13845 from crdlc/bug-940432

Bug 940432 - [Download Manager] [DownloadLauncher] Discover mime type fr...

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https://hg.mozilla.org/integration/gaia-central/rev/dc50f0c9f05a
Author: crdlc <crdlc@tid.es>
Desc: Bug 940432 - [Download Manager] [DownloadLauncher] Discover mime type from file extenstion