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

========

https://hg.mozilla.org/integration/gaia-central/rev/83c708eba811
Author: James Lal <james@lightsofapollo.com>
Desc: Merge pull request #12951 from wanderview/uitest-symlink-b2g

Bug 928631: support sym linked b2g in gaia-ui-test

========

https://hg.mozilla.org/integration/gaia-central/rev/48999903b977
Author: Ben Kelly <ben@wanderview.com>
Desc: Bug 928631: support sym linked b2g in gaia-ui-test