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

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https://hg.mozilla.org/integration/gaia-central/rev/15b9a5231e9e
Author: John Hu <johu@mozilla.com>
Desc: Merge pull request #11161 from huchengtw-moz/media/Bug_891797-2

Bug 891797 - [OTA][Data Migration][MediaDB] Data of Gallery, Video, and ...r=djf

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https://hg.mozilla.org/integration/gaia-central/rev/17e4246eac8d
Author: John Hu <johu@mozilla.com>
Desc: Bug 891797 - [OTA][Data Migration][MediaDB] Data of Gallery, Video, and Music app is different than original after update from v1.0.1 to v1.1.0.

b2g/config/gaia.json

@@ -1,4 +1,4 @@
 {
-    "revision": "72f1b7c657c65389c843dcd032e5bb787f4afafc", 
+    "revision": "15b9a5231e9e1d128f1fa1f520123f9380b5b1f1", 
     "repo_path": "/integration/gaia-central"
 }