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

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https://hg.mozilla.org/integration/gaia-central/rev/5b12854ab48e
Author: Ben Kelly <bkelly@mozilla.com>
Desc: Merge pull request #15748 from wanderview/ogg-to-opus

Bug 964584: Convert vorbis files to opus to improve memory usage. r=timdream

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https://hg.mozilla.org/integration/gaia-central/rev/1f02b5acaea2
Author: Ben Kelly <ben@wanderview.com>
Desc: Bug 964584: Convert vorbis files to opus to improve memory usage.
Bumping manifests a=b2g-bump