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

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https://hg.mozilla.org/integration/gaia-central/rev/6ac9087ee6e6
Author: Julien Wajsberg <felash@gmail.com>
Desc: Bug 1073157 - [System] fix wallpaper_manager.js tests with a newer mocha r=djf
Bumping manifests a=b2g-bump