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

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https://hg.mozilla.org/integration/gaia-central/rev/6b9ad7eff499
Author: Zac Campbell <zcampbell@mozilla.com>
Desc: Bug 959344 - Disable on TBPL for mac only test_settings_gps.py a=testonly