Bumping gaia.json for 1 gaia-central revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/207a81a6816b
Author: James Lal <james@lightsofapollo.com>
Desc: Bug 930500 - Many builds error because they can't find a binary, Bug 935489 - mozilla-download should always display the downloaded URL

Bump up mozilla-download version