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

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https://hg.mozilla.org/integration/gaia-central/rev/4ff387ba1fbb
Author: Eitan Isaacson <eitan@monotonous.org>
Desc: Merge pull request #12282 from eeejay/bug-917586

Bug 917586 - Use newer version of screen reader simulator in desktop firefox. r=kgrandon

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https://hg.mozilla.org/integration/gaia-central/rev/467adffea105
Author: Eitan Isaacson <eitan@monotonous.org>
Desc: Bug 917586 - Use newer version of screen reader simulator in desktop firefox.

b2g/config/gaia.json

@@ -1,4 +1,4 @@
 {
-    "revision": "a5b50081635b37c6f2bcfbf4b533a21996768f5c", 
+    "revision": "4ff387ba1fbbbedcf4930e3381232a4f6b8b9624", 
     "repo_path": "/integration/gaia-central"
 }