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

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https://hg.mozilla.org/integration/gaia-central/rev/4b5986581fb2
Author: Eitan Isaacson <eitan@monotonous.org>
Desc: Merge pull request #16051 from yzen/957674

Bug 957674 - adding an easy way to enable/disable a screen reader. r=alive

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https://hg.mozilla.org/integration/gaia-central/rev/8f45be5883eb
Author: Yura Zenevich <yzenevich@mozilla.com>
Desc: Bug 957674 - adding an easy way to enable/disable a screen reader.

b2g/config/gaia.json

@@ -1,4 +1,4 @@
 {
-    "revision": "64b67a585ea1d9adbaff155defde0fb2da5410e8", 
+    "revision": "4b5986581fb2df563af37b7290d4965a71accc80", 
     "repo_path": "/integration/gaia-central"
 }