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

========

https://hg.mozilla.org/integration/gaia-central/rev/6aa0e53d6672
Author: Rex KM Lee <rexboy@mozilla.com>
Desc: Merge pull request #10910 from rexboy7/fixhd-881146-ftu-master

Bug 881146 - hdpi/xhdpi resolution assets and layout for FTU. r=arcturus

========

https://hg.mozilla.org/integration/gaia-central/rev/4f8fc20dae05
Author: Rex Lee <rexboy@mozilla.com>
Desc: Bug 881146 - hdpi/xhdpi resolution assets and layout for First run experience app