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

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https://hg.mozilla.org/integration/gaia-central/rev/c9d3a8f7aa91
Author: Ben Kelly <bkelly@mozilla.com>
Desc: Merge pull request #12835 from wanderview/shared-tvm-append

Bug 918179: Optimize append case in tag_visibility_monitor.  r=djf

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https://hg.mozilla.org/integration/gaia-central/rev/28a0602cf111
Author: Ben Kelly <ben@wanderview.com>
Desc: Bug 918179: Optimize append case in tag_visibility_monitor