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

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https://hg.mozilla.org/integration/gaia-central/rev/18107faf6f15
Author: George <georgeiscoming@gmail.com>
Desc: Merge pull request #12679 from cctuan/922964

Bug 922964 - put the preload iframe farer

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https://hg.mozilla.org/integration/gaia-central/rev/6f2342761cf5
Author: cctuan <georgeiscoming@gmail.com>
Desc: Bug 922964 - put the preload iframe farer