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

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https://hg.mozilla.org/integration/gaia-2_0/rev/63851b559276
Author: Evelyn Hung <jj.evelyn@gmail.com>
Desc: Bug 1044960 - hide startscreen if tab url was null
Bumping manifests a=b2g-bump