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

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https://hg.mozilla.org/integration/gaia-central/rev/e18a5c70c22e
Author: Zac Campbell <zcampbell@mozilla.com>
Desc: Bug 952134 - Update requirements file to use Marionette 0.7.2 r=me