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

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https://hg.mozilla.org/integration/gaia-central/rev/8bb5b636e08b
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Revert "Bug 971654 - Correct lockscreen background sizing"

This reverts commit 21d0c2d77314325e7330daf4ebb90ed844a0c9ed.
Bumping manifests a=b2g-bump