Bumping gaia.json for 3 gaia-central revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/8b9ce757677e
Author: Ryan VanderMeulen <rvandermeulen@mozilla.com>
Desc: Merge pull request #13648 from yzen/937859

Bug 937859 - Making emergency callback markup a11y friendly.

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https://hg.mozilla.org/integration/gaia-central/rev/8dbea64ce813
Author: Yura Zenevich <yura.zenevich@gmail.com>
Desc: Bug 937859 - Making emergency callback markup a11y friendly.

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https://hg.mozilla.org/integration/gaia-central/rev/6b157bcc5cb6
Author: zacc <zcampbell@mozilla.com>
Desc: Bug 915138 - Xfail python test test_cards_view_with_three_apps.py