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

========

https://hg.mozilla.org/integration/gaia-central/rev/7b2351598d35
Author: Pavel Ivanov <pivanov@mozilla.com>
Desc: Merge pull request #21733 from pivanov/bug-1035731

Bug 1035731 - [Shared] - remove unused images

========

https://hg.mozilla.org/integration/gaia-central/rev/d7227ce02f4e
Author: Pavel Ivanov <pivanov@mozilla.com>
Desc: Bug 1035731 - [Shared] - remove unused images
Bumping manifests a=b2g-bump