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

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https://hg.mozilla.org/integration/gaia-central/rev/5cb856a484c8
Author: Etienne Segonzac <etienne@segonzac.info>
Desc: Merge pull request #12728 from alivedise/bugzilla/924233_master/bm_test_fail

Bug 924233 - Fix Battery manager unit test r=etienne

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https://hg.mozilla.org/integration/gaia-central/rev/fb21c91b76cf
Author: Alive Kuo <alegnadise@gmail.com>
Desc: Bug 924233 - Fix broken unit test in system