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

========

https://hg.mozilla.org/integration/gaia-central/rev/7b6faaaa6e24
Author: David Flanagan <dflanagan@mozilla.com>
Desc: Merge pull request #20410 from amitav-anand/backbtn_1016255

Bug 1016255 - Video played from Gallery goes back to gallery view on Pre... r=djf

========

https://hg.mozilla.org/integration/gaia-central/rev/4de7a3a663e9
Author: amitav-anand <amitav.anand@accenture.com>
Desc: Bug 1016255 - Video played from Gallery goes back to gallery view on Pressing left top corner,no back button visible though ;r=reviews
Bumping manifests a=b2g-bump