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

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https://hg.mozilla.org/integration/gaia-central/rev/154bb18c48ff
Author: Alive.Kuo <alegnadise@gmail.com>
Desc: Merge pull request #12785 from alivedise/bugzilla/908601_master_v2/default-orientation

Bug 908601 - Support default orientation, r=timdream, a=koi+

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https://hg.mozilla.org/integration/gaia-central/rev/d18785c1ca4c
Author: Alive Kuo <alegnadise@gmail.com>
Desc: Bug 908601 - Support default orientation in system app