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

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https://hg.mozilla.org/integration/gaia-central/rev/13d91fc76332
Author: RickyChien <ricky060709@gmail.com>
Desc: Merge pull request #16891 from RickyChien/bug-972699

Bug 972699 - Fix unit test zero coverage when using lazy load r=@yurenju

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https://hg.mozilla.org/integration/gaia-central/rev/20cf28b60560
Author: Ricky Chien <ricky060709@gmail.com>
Desc: Bug 972699 - Fix unit test zero coverage when using lazy load

b2g/config/gaia.json

@@ -4,6 +4,6 @@
         "branch": "", 
         "revision": ""
     }, 
-    "revision": "b3a73b86f97702ac3a633b8767e597d06ee1f825", 
+    "revision": "13d91fc76332616d7bd46cb310d1769c0da92ccc", 
     "repo_path": "/integration/gaia-central"
 }