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

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https://hg.mozilla.org/integration/gaia-central/rev/83c708eba811
Author: James Lal <james@lightsofapollo.com>
Desc: Merge pull request #12951 from wanderview/uitest-symlink-b2g

Bug 928631: support sym linked b2g in gaia-ui-test

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https://hg.mozilla.org/integration/gaia-central/rev/48999903b977
Author: Ben Kelly <ben@wanderview.com>
Desc: Bug 928631: support sym linked b2g in gaia-ui-test

b2g/config/gaia.json

@@ -1,4 +1,4 @@
 {
-    "revision": "c79ce4a07c3fdcfa4fa2dfdafe022d4791d87e77", 
+    "revision": "83c708eba811e47844c67f9f4ecc5a890f1273b5", 
     "repo_path": "/integration/gaia-central"
 }