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

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https://hg.mozilla.org/integration/gaia-central/rev/63b137e2b986
Author: vingtetun <21@vingtetun.org>
Desc: Merge pull request #15995 from vingtetun/bug967878

Bug 967878 - [Messages] Background doesn't scroll with the threads causi...

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https://hg.mozilla.org/integration/gaia-central/rev/3282fcb707c5
Author: Vivien Nicolas <21@vingtetun.org>
Desc: Bug 967878 - [Messages] Background doesn't scroll with the threads causing expensive over-invalidation. r=felash,bgirard

b2g/config/gaia.json

@@ -1,4 +1,4 @@
 {
-    "revision": "a0c2339eb78b70f98f08eb44f2f97c2de3cce4fe", 
+    "revision": "63b137e2b986a2ffa128ed506e389b6422bd324e", 
     "repo_path": "/integration/gaia-central"
 }