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

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https://hg.mozilla.org/integration/gaia-central/rev/d9c0618194e0
Author: Eitan Isaacson <eitan@monotonous.org>
Desc: Merge pull request #16615 from eeejay/bug-976215

Bug 976215 - [System] Make utility tray invisible. r=21

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https://hg.mozilla.org/integration/gaia-central/rev/72f0e4ced112
Author: Eitan Isaacson <eitan@monotonous.org>
Desc: Bug 976215 - [System] Make utility tray invisible.

b2g/config/gaia.json

@@ -1,4 +1,4 @@
 {
-    "revision": "647c6e6a226f94f24750fca01616181adf71b496", 
+    "revision": "d9c0618194e0272b7dc57dc9dfd1787adc542c5b", 
     "repo_path": "/integration/gaia-central"
 }