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

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https://hg.mozilla.org/integration/gaia-central/rev/481bc5b521cd
Author: steveck-chung <schung@mozilla.com>
Desc: Merge pull request #11209 from steveck-chung/hdpi-image-assets

Bug 898299 - hdpi/xhdpi Image assets polishing in Message app. r=rexboy

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https://hg.mozilla.org/integration/gaia-central/rev/680657d97a21
Author: Steve Chung <schung@mozilla.com>
Desc: Bug 898299 - hdpi/xhdpi Image assets polishing in Message app

b2g/config/gaia.json

@@ -1,4 +1,4 @@
 {
-    "revision": "c2a13d1a22786ba0c9d1be6a9791e2db09ba4983", 
+    "revision": "481bc5b521cd538c68f442f402d31f9ae3b73959", 
     "repo_path": "/integration/gaia-central"
 }