Bumping gaia.json for 3 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/74a9caf001bf
Author: Mason Chang <mason@masonchang.com>
Desc: Merge pull request #16806 from changm/bug979018

Bug 979018 Make background scroll with content in SMS. r=julienw

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https://hg.mozilla.org/integration/gaia-central/rev/9a6e89241914
Author: Mason Chang <mchang@mozilla.com>
Desc: fix redundant background colors

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https://hg.mozilla.org/integration/gaia-central/rev/01ff2abf369a
Author: Mason Chang <mchang@mozilla.com>
Desc: Make background scroll with content in SMS
Bumping manifests a=b2g-bump