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

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https://hg.mozilla.org/integration/gaia-central/rev/6193987d46a8
Author: Timothy Guan-tin Chien <timdream@gmail.com>
Desc: Merge pull request #15889 from timdream/lockscreen-transitionz

Bug 966711 - Use translateZ() for lockscreen fade-out transition, r=snowmantw

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https://hg.mozilla.org/integration/gaia-central/rev/cf9b6c5f6372
Author: Timothy Guan-tin Chien <timdream@gmail.com>
Desc: Bug 966711 - Use translateZ() for lockscreen fade-out transition

b2g/config/gaia.json

@@ -1,4 +1,4 @@
 {
-    "revision": "e884c15f619d8793a82158f748e57dcce94795f3", 
+    "revision": "6193987d46a8d80c92554de0aa96462dd2a443b2", 
     "repo_path": "/integration/gaia-central"
 }