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

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https://hg.mozilla.org/integration/gaia-central/rev/55f131078d8d
Author: Cristian Rodriguez <crdlc@tid.es>
Desc: Merge pull request #21325 from crdlc/bug-1031890-lazyload

Bug 1031890 - [Lazy Load] Vertical homescreen takes a long time to load

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https://hg.mozilla.org/integration/gaia-central/rev/b151bb0b60be
Author: crdlc <crdlc@tid.es>
Desc: Bug 1031890 - [Lazy Load] Vertical homescreen takes a long time to load

b2g/config/gaia.json

@@ -4,6 +4,6 @@
         "remote": "", 
         "branch": ""
     }, 
-    "revision": "556fb9489256b2ce03430c25efb4a31879fe1671", 
+    "revision": "55f131078d8d72ab9b712cf9b6050ba31b950fc1", 
     "repo_path": "/integration/gaia-central"
 }