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

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https://hg.mozilla.org/integration/gaia-central/rev/903a93f5f678
Author: BavarianTomcat <tomcat@mozilla.com>
Desc: Revert "Bug 1022728 - Prevent building gaia when running ./build.sh gecko" for bustage

This reverts commit 2a16253b365d011f27e44a6fdc62a5c28f903598.
Bumping manifests a=b2g-bump