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

========

https://hg.mozilla.org/integration/gaia-central/rev/17a7e26d46c0
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Merge pull request #13852 from bgirard/HomescrenBackground

Bug 917416 - Move wallpaper into the Homescreen app. r=gal

========

https://hg.mozilla.org/integration/gaia-central/rev/fd764fab84f1
Author: Benoit Girard <b56girard@gmail.com>
Desc: Bug 917416 - Move wallpaper into the Homescreen app. r=gal