Bumping gaia.json for 1 gaia-central revision(s) a=gaia-bump

========

https://hg.mozilla.org/integration/gaia-central/rev/6b9ad7eff499
Author: Zac Campbell <zcampbell@mozilla.com>
Desc: Bug 959344 - Disable on TBPL for mac only test_settings_gps.py a=testonly

b2g/config/gaia.json

@@ -1,4 +1,4 @@
 {
-    "revision": "6b344043dfb5bf510f4c8caa1547c544c2fe1dca", 
+    "revision": "6b9ad7eff499d198092be6c2439e1034335589db", 
     "repo_path": "/integration/gaia-central"
 }