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

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https://hg.mozilla.org/integration/gaia-central/rev/6c4394feb826
Author: George <georgeiscoming@gmail.com>
Desc: Merge pull request #13579 from cctuan/907951

Bug 907951 - [System] replace forward images of wrapper

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https://hg.mozilla.org/integration/gaia-central/rev/1b921db798f8
Author: cctuan <georgeiscoming@gmail.com>
Desc: Bug 907951 - [System] replace forward images of wrapper