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

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https://hg.mozilla.org/integration/gaia-central/rev/912e2a8c3052
Author: Ryan VanderMeulen <ryanvm@gmail.com>
Desc: Merge pull request #26720 from aosmond/bug1104913

Bug 1104913 - Change app to rely upon optimal default selection chosen by gecko.

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https://hg.mozilla.org/integration/gaia-central/rev/b431894e9001
Author: Andrew Osmond <aosmond@mozilla.com>
Desc: Bug 1104913 - Change app to rely upon optimal default selection chosen by gecko.
Bumping manifests a=b2g-bump