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

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https://hg.mozilla.org/integration/gaia-central/rev/5599c50ef6bc
Author: Mike Pennisi <mike@mikepennisi.com>
Desc: Bug 972526 - [clock] Integration tests' Stopwatch abstraction hides a race condition

r=mcav
Bumping manifests a=b2g-bump