Bumping gaia.json for 4 gaia-central revision(s) a=gaia-bump

========

https://hg.mozilla.org/integration/gaia-central/rev/6e60cab7d94a
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Merge pull request #12780 from KevinGrandon/bug_921637_tooling_remove_activities_ext

Bug 921637 - Remove activities extension r=vingtetun

========

https://hg.mozilla.org/integration/gaia-central/rev/e2f71542b2da
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Bug 921637 - Remove activities code except ActivitiesGlue.js

========

https://hg.mozilla.org/integration/gaia-central/rev/8b59f4738a2d
Author: Zac <zcampbell@mozilla.com>
Desc: Merge pull request #13625 from bitgeeky/bug930992

Bug 930992 - Create a System app region for app install confirmation r=z...

========

https://hg.mozilla.org/integration/gaia-central/rev/83bf78658248
Author: bitgeeky <mozpankaj1994@gmail.com>
Desc: Bug 930992 - Create a System app region for app install confirmation r=zcampbell@mozilla.com