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

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https://hg.mozilla.org/integration/gaia-central/rev/553715baac40
Author: Jonathan Griffin <jgriffin@mozilla.com>
Desc: Merge pull request #11407 from ahal/xulrunner

Bug 902496 - Update copy of xulrunner-sdk being used

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https://hg.mozilla.org/integration/gaia-central/rev/8d6106f716c6
Author: Andrew Halberstadt <halbersa@gmail.com>
Desc: Bug 902496 - Update copy of xulrunner-sdk being used, r=jgriffin