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

========

https://hg.mozilla.org/integration/gaia-central/rev/b18d7bedcb94
Author: Zac <zcampbell@mozilla.com>
Desc: Merge pull request #12588 from bebef1987/wifi_issue

Bug 922702 - Investigate test_connect_to_wifi_via_settings_app r=zac

========

https://hg.mozilla.org/integration/gaia-central/rev/3dd859087021
Author: Bebe <bebe@mozilla.ro>
Desc: Bug 922702 - Investigate test_connect_to_wifi_via_settings_app