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

========

https://hg.mozilla.org/integration/gaia-central/rev/721762333dd7
Author: Borja Salguero <borjasalguero@users.noreply.github.com>
Desc: Merge pull request #13905 from borjasalguero/shield_callback_code

Bug 941597 - [Regression] Error when using activity from contacts r=juli...

========

https://hg.mozilla.org/integration/gaia-central/rev/b5a6fbdc6a09
Author: borjasalguero <fbsc@tid.es>
Desc: Bug 941597 - [Regression] Error when using activity from contacts r=julienw
Bug 937074 - [Bluetooth] Make sure SLC established before we set up SCO. r=echou