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

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https://hg.mozilla.org/integration/gaia-central/rev/b6f13ded220d
Author: Gareth Aye <gaye@mozilla.com>
Desc: Merge pull request #14908 from gaye/bug-952611

Bug 952611 - Disable broken gaia integration tests

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https://hg.mozilla.org/integration/gaia-central/rev/3db9d411c1b2
Author: Gareth Aye <gaye@mozilla.com>
Desc: Bug 952611 - Disable broken gaia integration tests