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

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https://hg.mozilla.org/integration/gaia-central/rev/a15a2479ef80
Author: Joan Leon <joan.leon@gmail.com>
Desc: Merge pull request #17386 from nucliweb/973827-Power-button-modal

Power-button-modal

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https://hg.mozilla.org/integration/gaia-central/rev/37cfe1536eaf
Author: Joan Leon <joan.leon@gmail.com>
Desc: Power-button-modal

b2g/config/gaia.json

@@ -4,6 +4,6 @@
         "branch": "", 
         "revision": ""
     }, 
-    "revision": "9fc2a79c615a72c15c995ffc342e574b342a6e60", 
+    "revision": "a15a2479ef80212bc8ce76cedf5eba5f176a9e51", 
     "repo_path": "/integration/gaia-central"
 }