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

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https://hg.mozilla.org/integration/gaia-central/rev/bc873f47546b
Author: BavarianTomcat <tomcat@mozilla.com>
Desc: Revert "Bug 971552 - [System2] Instantiable SleepMenu r=alive" for gaia unit test failures

This reverts commit c20eb50c5232874009cc3f981ad63a62e6efa0b5.
Bumping manifests a=b2g-bump