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

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https://hg.mozilla.org/integration/gaia-central/rev/6a734adb5550
Author: Greg Weng <snowmantw@gmail.com>
Desc: Merge pull request #12496 from snowmantw/issue921360

Bug 921360 - [LockScreen] Make the glitches on lockscreen invisible

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https://hg.mozilla.org/integration/gaia-central/rev/d687f3ffd87a
Author: Greg Weng (snowmantw) <gweng@mozilla.com>
Desc: Bug 921360 - [LockScreen] Make the glitches on lockscreen invisible