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

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https://hg.mozilla.org/integration/gaia-2_0/rev/8b78a238248e
Author: Zac <zcampbell@mozilla.com>
Desc: Bug 1020342 - Update marionette client in gaiatest to 0.7.9
Bumping manifests a=b2g-bump