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

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https://hg.mozilla.org/integration/gaia-central/rev/4a67b4e4f948
Author: lissyx <lissyx+github@lissyx.dyndns.org>
Desc: Merge pull request #18709 from lissyx/bug963234

Bug 963234 - Switch to mozNotificationEvent r=mhenretty,alive

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https://hg.mozilla.org/integration/gaia-central/rev/c91eff6ed29f
Author: Alexandre Lissy <lissyx+github@lissyx.dyndns.org>
Desc: Bug 963234 - Switch to mozNotificationEvent r=mhenretty,alive

b2g/config/gaia.json

@@ -4,6 +4,6 @@
         "remote": "", 
         "branch": ""
     }, 
-    "revision": "aff973323f602884d0f43b07cc856d67dc0f058d", 
+    "revision": "4a67b4e4f948392acfef37069392193a4bc3b0f6", 
     "repo_path": "/integration/gaia-central"
 }