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

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https://hg.mozilla.org/integration/gaia-central/rev/50018d17d90a
Author: Zac <zcampbell@mozilla.com>
Desc: Merge pull request #16673 from malini/vbump

Bug 974600 - update to marionette_client 0.7.5, bump version

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https://hg.mozilla.org/integration/gaia-central/rev/d6f42816c982
Author: Malini Das <mdas@mozilla.com>
Desc: update to marionette_client 0.7.5, bump version
Bumping manifests a=b2g-bump