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

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https://hg.mozilla.org/integration/gaia-central/rev/6f9fc4eefca5
Author: Mason Chang <mason@masonchang.com>
Desc: Merge pull request #15367 from changm/bug950250

Bug 950250 Make background scroll with content r=arthurcc

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https://hg.mozilla.org/integration/gaia-central/rev/60100c1e3c86
Author: Mason Chang <mchang@mozilla.com>
Desc: Bug 950250 Make background scroll with content r=arthurcc