Bumping gaia.json for 2 gaia revision(s) a=gaia-bump

========

https://hg.mozilla.org/integration/gaia-central/rev/6195441c7748
Author: Etienne Segonzac <etienne@segonzac.info>
Desc: Merge pull request #17087 from etiennesegonzac/bug-980984-incoming-alert-from-system

Bug 980984 - Alert the user of an incoming from the system app to improve performance r=alive, rik

========

https://hg.mozilla.org/integration/gaia-central/rev/b15ab79f660a
Author: Etienne Segonzac <etienne@segonzac.info>
Desc: Bug 980984 - Alert the user of an incoming from the system app to improve performance.
r=alive, rik
Bumping manifests a=b2g-bump