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

========

https://hg.mozilla.org/integration/gaia-central/rev/8e9d343ad39f
Author: vingtetun <21@vingtetun.org>
Desc: Merge pull request #16767 from vingtetun/bug978565

Bug 978565 - Add will-change css rule to the homescreen. r=bgirard

========

https://hg.mozilla.org/integration/gaia-central/rev/a6b30851cdd4
Author: Vivien Nicolas <21@vingtetun.org>
Desc: Bug 978565 - Add will-change css rule to the homescreen. r=bgirard
Bumping manifests a=b2g-bump