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

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https://hg.mozilla.org/integration/gaia-central/rev/c6bd218f1252
Author: Zac Campbell <zcampbell@mozilla.com>
Desc: Bug 922062 - Skip cleanUp steps that are not necessary when --restart is passed in. r=bebe r=davehunt