Bumping gaia.json for 3 gaia revision(s) a=gaia-bump

========

https://hg.mozilla.org/integration/gaia-central/rev/a220240dbe41
Author: Chris Lord <chrislord.net@gmail.com>
Desc: Merge pull request #25123 from Cwiiis/bug1061466-system-update-splash

Bug 1061466 - Show a splash screen when doing a system update. r=alive

========

https://hg.mozilla.org/integration/gaia-central/rev/bb315afca127
Author: Chris Lord <chrislord.net@gmail.com>
Desc: Bug 1061466 - Add a basic marionette test for update splash-screen. r=alive

========

https://hg.mozilla.org/integration/gaia-central/rev/91969023d725
Author: Chris Lord <chrislord.net@gmail.com>
Desc: Bug 1061466 - Show a splash screen when doing a system update. r=alive
Bumping manifests a=b2g-bump