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

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https://hg.mozilla.org/integration/gaia-central/rev/e121aac89613
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Bug 1004768 - [Home2] Remove will-change property as it hurts scrolling performance with too many individual layers r=me
Bumping manifests a=b2g-bump