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

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https://hg.mozilla.org/integration/gaia-central/rev/b4923337d01a
Author: dwi2 <dwi2@dwi2.com>
Desc: Merge pull request #18116 from dwi2/bug975293

Bug 975293 - Make sure we have _containerElement defined in the context ...

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https://hg.mozilla.org/integration/gaia-central/rev/4ea68c15cd9f
Author: Tzu-Lin Huang <tzhuang@mozilla.com>
Desc: Bug 975293 - Make sure we have _containerElement defined in the context of _onTransitionEnd
Bumping manifests a=b2g-bump