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

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https://hg.mozilla.org/integration/gaia-central/rev/4a15e825a821
Author: J. Ryan Stinnett <jryans@gmail.com>
Desc: Merge pull request #18907 from jryans/system-dialog-sim

Bug 1005417 - Fix bottom space below system dialogs in Simulator

========

https://hg.mozilla.org/integration/gaia-central/rev/68ec8821be6e
Author: J. Ryan Stinnett <jryans@gmail.com>
Desc: Bug 1005417 - Fix bottom space below system dialogs in Simulator. r=alive

b2g/config/gaia.json

@@ -4,6 +4,6 @@
         "remote": "", 
         "branch": ""
     }, 
-    "revision": "f1699bbdaec19da2102d049f38590483e20f50f5", 
+    "revision": "4a15e825a8211bec0ed59d2bfcef914a3032b5c9", 
     "repo_path": "/integration/gaia-central"
 }