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

========

https://hg.mozilla.org/integration/gaia-central/rev/553715baac40
Author: Jonathan Griffin <jgriffin@mozilla.com>
Desc: Merge pull request #11407 from ahal/xulrunner

Bug 902496 - Update copy of xulrunner-sdk being used

========

https://hg.mozilla.org/integration/gaia-central/rev/8d6106f716c6
Author: Andrew Halberstadt <halbersa@gmail.com>
Desc: Bug 902496 - Update copy of xulrunner-sdk being used, r=jgriffin

b2g/config/gaia.json

@@ -1,4 +1,4 @@
 {
-    "revision": "acd53f668718d06090805303ed85e36dddf61853", 
+    "revision": "553715baac409aa4f12fee8413450986341c672e", 
     "repo_path": "/integration/gaia-central"
 }