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

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https://hg.mozilla.org/integration/gaia-central/rev/6fbf376b9813
Author: Gabriele Svelto <gsvelto@mozilla.com>
Desc: Merge pull request #16131 from gabrielesvelto/bug-917193-tones-glitch-workaround

Bug 917193 - Use samples instead of waveform generators for short audible tones

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https://hg.mozilla.org/integration/gaia-central/rev/b9b205d4a863
Author: Gabriele Svelto <gsvelto@mozilla.com>
Desc: Bug 917193 - Use samples instead of waveform generators for short audible tones r=etienne
Bumping manifests a=b2g-bump