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

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https://hg.mozilla.org/integration/gaia-central/rev/c921386cba61
Author: Sam Foster <sam@sam-i-am.com>
Desc: Merge pull request #20506 from sfoster/bug-1019289-videos

Bug 1019289 - Update FTU tutorial videos. r=me

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https://hg.mozilla.org/integration/gaia-central/rev/2fe189cd6783
Author: Sam Foster <sfoster@mozilla.com>
Desc: Bug 1019289 - Update FTU tutorial videos. r=me
Bumping manifests a=b2g-bump