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

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https://hg.mozilla.org/integration/gaia-central/rev/507f903f0254
Author: Mason Chang <mason@masonchang.com>
Desc: Merge pull request #14283 from changm/bug943089

bug 943089 keep scrolling portion of gallery app as one layer using will...

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https://hg.mozilla.org/integration/gaia-central/rev/43bb219195ed
Author: Mason Chang <mchang@mozilla.com>
Desc: bug 943089 keep scrolling portion of gallery app as one layer using will-animate