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

========

https://hg.mozilla.org/integration/gaia-central/rev/6a1273aa9942
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Merge pull request #15496 from KevinGrandon/bug_958582_follow_up_pointer_events

Bug 958582 - Follow-up, pointer-events fix r=me

========

https://hg.mozilla.org/integration/gaia-central/rev/578a91da7d9d
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Bug 958582 - Follow-up, pointer-events fix r=me
Bumping manifests a=b2g-bump