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

========

https://hg.mozilla.org/integration/gaia-central/rev/648de7b0cc94
Author: punamdahiya <punamdahiya@yahoo.com>
Desc: Merge pull request #21985 from punamdahiya/Bug1035492-Wallpaper

Bug1035492-[Wallpaper]-Use Img element instead of background images for gecko to handle decoded image memory r=djf

========

https://hg.mozilla.org/integration/gaia-central/rev/db99b084b1bc
Author: Punam Dahiya <punamdahiya@yahoo.com>
Desc: Bug1035492-[Wallpaper]-Use Img element instead of background images for gecko to handle decoded image memory r=djf
Bumping manifests a=b2g-bump