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

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https://hg.mozilla.org/integration/gaia-central/rev/3f6ed55451da
Author: Francisco Borja Salguero Castellano <fbsc@tid.es>
Desc: Merge pull request #10666 from steveck-chung/bug-887216

Bug 887216 - [MMS] The header in a group message is active, it works as the go back key

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https://hg.mozilla.org/integration/gaia-central/rev/603664b50503
Author: Steve Chung <schung@mozilla.com>
Desc: Bug 887216 - [MMS] The header in a group message is active, it works as the go back key

b2g/config/gaia.json

@@ -1,4 +1,4 @@
 {
-    "revision": "e70e23fc4b7a06caae301503c661b1093f1fb0ea", 
+    "revision": "3f6ed55451da1f960605f589fe383ed8c3ba807e", 
     "repo_path": "/integration/gaia-central"
 }