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

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https://hg.mozilla.org/integration/gaia-central/rev/518a7e03111f
Author: Jan Jongboom <janjongboom@gmail.com>
Desc: Merge pull request #10919 from comoyo/hidden_apps

Bug 892397 - Specify hidden apps in their respective manifests. r=fabrice

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https://hg.mozilla.org/integration/gaia-central/rev/fbc84847793e
Author: Jan Jongboom <janjongboom@gmail.com>
Desc: Bug 892397 - Specify hidden apps in their respective manifests

b2g/config/gaia.json

@@ -1,4 +1,4 @@
 {
-    "revision": "a815ebbbe084e5c278f65c73f183b89194e3833e", 
+    "revision": "518a7e03111f63020439e7d5ab428615deb126e2", 
     "repo_path": "/integration/gaia-central"
 }