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

========

https://hg.mozilla.org/integration/gaia-central/rev/5231b63ec62a
Author: Fabrice Desre <fabrice@desre.org>
Desc: Bug 968957 - Disable 3rd party keyboards until we make them work with Nuwa processes r=me
Bumping manifests a=b2g-bump