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

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https://hg.mozilla.org/integration/gaia-central/rev/f76db748e12b
Author: Anthony Ricaud <anthony@ricaud.me>
Desc: Merge pull request #11853 from Rik/attention-screen-size-908186

Bug 908186 - Listen to mozbrowserresize events to hide or show the atten... r=etienne

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https://hg.mozilla.org/integration/gaia-central/rev/564bd1d934b1
Author: Anthony Ricaud <anthony@ricaud.me>
Desc: Bug 908186 - Listen to mozbrowserresize events to hide or show the attention screen

b2g/config/gaia.json

@@ -1,4 +1,4 @@
 {
-    "revision": "22d72f1cfb077cafd03cd4f879acffdaca5347e5", 
+    "revision": "f76db748e12b4d8de4da9921d0ee7756f14d9a54", 
     "repo_path": "/integration/gaia-central"
 }