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

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https://hg.mozilla.org/integration/gaia-central/rev/6a8c178c6174
Author: Zac <zcampbell@mozilla.com>
Desc: Merge pull request #15009 from zacc/bug_949897

Bug 949897 - Investigate intermittents in test_cards_view_kill_apps_with_two_apps - Reenable test

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https://hg.mozilla.org/integration/gaia-central/rev/7d6d996662f6
Author: Zac Campbell <zcampbell@mozilla.com>
Desc: Bug 949897 - Investigate intermittents in test_cards_view_kill_apps_with_two_apps - Reenable test
Bumping manifests a=b2g-bump