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

========

https://hg.mozilla.org/integration/gaia-central/rev/4a15e825a821
Author: J. Ryan Stinnett <jryans@gmail.com>
Desc: Merge pull request #18907 from jryans/system-dialog-sim

Bug 1005417 - Fix bottom space below system dialogs in Simulator

========

https://hg.mozilla.org/integration/gaia-central/rev/68ec8821be6e
Author: J. Ryan Stinnett <jryans@gmail.com>
Desc: Bug 1005417 - Fix bottom space below system dialogs in Simulator. r=alive
Bumping manifests a=b2g-bump