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

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https://hg.mozilla.org/integration/gaia-central/rev/855e3dca520a
Author: Zac <zcampbell@mozilla.com>
Desc: Merge pull request #20363 from zacc/bug_1020342

Bug 1020342 - Update marionette client in gaiatest to 0.7.9

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https://hg.mozilla.org/integration/gaia-central/rev/600a85c04bcf
Author: Zac <zcampbell@mozilla.com>
Desc: Bug 1020342 - Update marionette client in gaiatest to 0.7.9
Bumping manifests a=b2g-bump