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

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https://hg.mozilla.org/integration/gaia-central/rev/d12e96f0eb91
Author: BavarianTomcat <tomcat@mozilla.com>
Desc: Revert "Bug 1029967 - rewrite Makefile in email app in javascript r=gduan" for windows b2g bustage

This reverts commit 320a7ad564dfb82b8a5cd5413e87dd44c39d9db7.
Bumping manifests a=b2g-bump