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

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https://hg.mozilla.org/integration/gaia-central/rev/6c394d3b633a
Author: Wilson Page <wilsonpage@me.com>
Desc: Merge pull request #18029 from wilsonpage/988670

Bug 988670 - [Camera][Madai] Behavior when leaving camera while in previ...

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https://hg.mozilla.org/integration/gaia-central/rev/87c0bf8371e6
Author: hyuna.cho <hyuna.cho@lge.com>
Desc: Bug 988670 - [Camera][Madai] Behavior when leaving camera while in preview mode and hitting home button needs to be reconsidered
Bumping manifests a=b2g-bump