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

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https://hg.mozilla.org/integration/gaia-central/rev/6ef94e486227
Author: John Hu <johu@mozilla.com>
Desc: Merge pull request #15689 from russnicoletti/bug833438

Bug 833438 - Viewing video through activity doesn't pause after headset ..., r=johnhu

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https://hg.mozilla.org/integration/gaia-central/rev/d157a76112cb
Author: Russ Nicoletti <rnicoletti@mozilla.com>
Desc: Bug 833438 - Viewing video through activity doesn't pause after headset is removed
Bumping manifests a=b2g-bump