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

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https://hg.mozilla.org/integration/gaia-2_0/rev/c39a6debdf8e
Author: Botond Ballo <botond@mozilla.com>
Desc: Bug 1064401 - Disable overscrolling completely for b2g 2.0 (gaia portion). r=kats, a=bajaj
Bumping manifests a=b2g-bump