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

========

https://hg.mozilla.org/integration/gaia-central/rev/721885a795cc
Author: Michael Henretty <michael.henretty@gmail.com>
Desc: Merge pull request #20183 from gmarty/gmarty-bug-983812-shiftedtests

Bug 983812 - Center header text based on amount of available space

========

https://hg.mozilla.org/integration/gaia-central/rev/a92410784054
Author: Guillaume Marty <edo999@gmail.com>
Desc: Bug 983812 - Center header text based on amount of available space
Bumping manifests a=b2g-bump