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

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https://hg.mozilla.org/integration/gaia-central/rev/3c8909adaa95
Author: Zac Campbell <zcampbell@mozilla.com>
Desc: Bug 959344 - Disable intermittent test_settings_gps on mac osx r=me a=testonly
Bug 957880 - Expose memory pressure events to gaia's system app r=gwagner