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

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https://hg.mozilla.org/integration/gaia-central/rev/6ec23e652d0f
Author: Zac <zcampbell@mozilla.com>
Desc: Merge pull request #12556 from elefant/system-message-test

Bug 768228 - Add System message test

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https://hg.mozilla.org/integration/gaia-central/rev/b320cf861b66
Author: hchang <hchang@mozilla.com>
Desc: Bug 768228 - Add System message test