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

========

https://hg.mozilla.org/integration/gaia-central/rev/3c8909adaa95
Author: Zac Campbell <zcampbell@mozilla.com>
Desc: Bug 959344 - Disable intermittent test_settings_gps on mac osx r=me a=testonly

b2g/config/gaia.json

@@ -1,4 +1,4 @@
 {
-    "revision": "a4f1842f7bd53360b3f92db1b3a44f6b8f8f589c", 
+    "revision": "3c8909adaa956991ff07daa72ab5d91e76dd1abd", 
     "repo_path": "/integration/gaia-central"
 }