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

========

https://hg.mozilla.org/integration/gaia-central/rev/518a7e03111f
Author: Jan Jongboom <janjongboom@gmail.com>
Desc: Merge pull request #10919 from comoyo/hidden_apps

Bug 892397 - Specify hidden apps in their respective manifests. r=fabrice

========

https://hg.mozilla.org/integration/gaia-central/rev/fbc84847793e
Author: Jan Jongboom <janjongboom@gmail.com>
Desc: Bug 892397 - Specify hidden apps in their respective manifests