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

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https://hg.mozilla.org/integration/gaia-central/rev/8eec3778c4ca
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Merge pull request #15378 from KevinGrandon/bug_960240_disable_persona_test

Bug 960240 - Disable intermittent failing test: test_persona_cookie.py

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https://hg.mozilla.org/integration/gaia-central/rev/3b286add275e
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Bug 960240 - Disable intermittent failing test: test_persona_cookie.py