Bumping gaia.json for 2 gaia-1_2 revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-1_2/rev/15a73747913b
Author: Aki Sasaki <aki@escapewindow.com>
Desc: Merge pull request #12765 from escapewindow/v1.2

bug 918320 - Add bg, bn-BD, it, mk, zh-CN to v1.2 builds. r=pike

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https://hg.mozilla.org/integration/gaia-1_2/rev/4db8b9d6fb70
Author: Aki Sasaki <aki@escapewindow.com>
Desc: bug 918320 - Add bg, bn-BD, it, mk, zh-CN to v1.2 builds