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

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https://hg.mozilla.org/integration/gaia-central/rev/e96012de6040
Author: Staś Małolepszy <stas@mozilla.com>
Desc: Merge pull request #18753 from stasm/1002498-ringtones-once

Bug 1002498 - Use mozL10n.once in Ringtones. r=dkuo

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https://hg.mozilla.org/integration/gaia-central/rev/631d033884e0
Author: Staś Małolepszy <stas@mozilla.com>
Desc: Bug 1002498 - Use mozL10n.once in Ringtones. r=dkuo
Bumping manifests a=b2g-bump