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

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https://hg.mozilla.org/integration/gaia-central/rev/3451eb6b72d2
Author: Zac Campbell <zcampbell@mozilla.com>
Desc: Bug 946812 - Disable test_camera_flash on Hamachi (no flash present) a=testonly r=me

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https://hg.mozilla.org/integration/gaia-central/rev/24822dd90ea8
Author: Zac Campbell <zcampbell@mozilla.com>
Desc: Bug 949897 - Disable test_cards_view_kill_apps_two_apps for device a=testonly r=me

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https://hg.mozilla.org/integration/gaia-central/rev/a8ac9ba099f3
Author: rwood-moz <rwood@mozilla.com>
Desc: Bug 964280 - Add option to override b2g start timeout duration. r=dhunt
Bumping manifests a=b2g-bump