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

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https://hg.mozilla.org/integration/gaia-central/rev/18b3a69398d2
Author: Zac <zcampbell@mozilla.com>
Desc: Merge pull request #13746 from bobsilverberg/bug939062

Bug 939062 - Improve wait in test_add_contact_to_favorites.py

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https://hg.mozilla.org/integration/gaia-central/rev/3a6dbe41269f
Author: Bob Silverberg <bob.silverberg@gmail.com>
Desc: Bug 939062 - Improve wait in test_add_contact_to_favorites.py