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

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https://hg.mozilla.org/integration/gaia-central/rev/a313d5e13d4f
Author: Alive.Kuo <alegnadise@gmail.com>
Desc: Merge pull request #12479 from alivedise/bugzilla/920944/two-homescreen-instance-in-b2gperf

Bug 920944 - Fix b2gperf, r=timdream

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https://hg.mozilla.org/integration/gaia-central/rev/fc8a0445f77b
Author: Alive Kuo <alegnadise@gmail.com>
Desc: Bug 920944 - Fix b2gperf