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

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https://hg.mozilla.org/integration/gaia-central/rev/c288983a9abc
Author: Ghislain 'Aus' Lacroix <aus@noiseport.org>
Desc: Merge pull request #20939 from nullaus/bug1027414

bug 1027414 - Dismiss on visibilitychange r=alive (on behalf of 21)

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https://hg.mozilla.org/integration/gaia-central/rev/a4227d40db5f
Author: Ghislain 'Aus' Lacroix <aus@mozilla.com>
Desc: bug 1027414 - Dismiss on visibilitychange r=alive (on behalf of 21)
Bumping manifests a=b2g-bump