Bumping gaia.json for 4 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/bff2db184624
Author: Gabriele Svelto <gsvelto@mozilla.com>
Desc: Merge pull request #30879 from gabrielesvelto/bug-1175384-emergency-call-header

Bug 1175384 - Make the emergency call app use the gaia themes r=etienne_s

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https://hg.mozilla.org/integration/gaia-central/rev/55d6c30fc450
Author: Gabriele Svelto <gsvelto@mozilla.com>
Desc: Bug 1175384 - Make the emergency call app use the gaia themes r=etienne_s

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https://hg.mozilla.org/integration/gaia-central/rev/08331c30fc43
Author: Jim Porter <jporter@mozilla.com>
Desc: Merge pull request #30768 from jimporter/music-lazyloader-promise

Bug 1179112 - [music] Use the Promisified version of the LazyLoader

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https://hg.mozilla.org/integration/gaia-central/rev/6b780565bfea
Author: Jim <jporter@mozilla.com>
Desc: Bug 1179112 - [music] Use the Promisified version of the LazyLoader

b2g/config/gaia.json

@@ -1,9 +1,9 @@
 {
     "git": {
-        "git_revision": "6e68c718c1173fd97e912c51000a4223056d36e6", 
+        "git_revision": "bde52446a18ba9842af7ebd238cd293e64a5d6f5", 
         "remote": "https://git.mozilla.org/releases/gaia.git", 
         "branch": ""
     }, 
-    "revision": "031af779537402407c1692ce1d9b03bc3841ed16", 
+    "revision": "bff2db184624627979dc99a581292118d19d3a70", 
     "repo_path": "integration/gaia-central"
 }