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

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https://hg.mozilla.org/integration/gaia-central/rev/3b83e12f8504
Author: Ben Kelly <bkelly@mozilla.com>
Desc: Merge pull request #12821 from wanderview/system-test-require

Bug 924233: Add missing require to system tests. r=etienne

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https://hg.mozilla.org/integration/gaia-central/rev/2df5bb5c5794
Author: Ben Kelly <ben@wanderview.com>
Desc: Bug 924233: Add missing require to system tests.