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

========

https://hg.mozilla.org/integration/gaia-central/rev/644d81b4e86c
Author: Zac <zcampbell@mozilla.com>
Desc: Bug 990580 - Workaround python launch() issue a=testonly r=me
Bumping manifests a=b2g-bump