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

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https://hg.mozilla.org/integration/gaia-central/rev/68cba47518ba
Author: Jose M. Cantera <jmcanterafonseca@gmail.com>
Desc: Merge pull request #12373 from wanderview/contacts-static-scrollbar

Bug 919692: define contacts scrollbar in index.html

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https://hg.mozilla.org/integration/gaia-central/rev/47ba5adbd985
Author: Ben Kelly <ben@wanderview.com>
Desc: Bug 919692: define contacts scrollbar in index.html

b2g/config/gaia.json

@@ -1,4 +1,4 @@
 {
-    "revision": "af2c025bbb504880ef58f777ee26cc23a250f299", 
+    "revision": "68cba47518ba1578b2c455cd47518b4087c7bd48", 
     "repo_path": "/integration/gaia-central"
 }