Bumping gaia.json for 1 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/ac7418660457
Author: Zac <zcampbell@mozilla.com>
Desc: Bug 1022132 - Disable test_a11y_unlock_to_homescreen.py on TBPL
Bumping manifests a=b2g-bump