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

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https://hg.mozilla.org/integration/gaia-central/rev/e15140483023
Author: Ghislain 'Aus' Lacroix <aus@noiseport.org>
Desc: Merge pull request #18698 from nullaus/bug999511

bug 999511

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https://hg.mozilla.org/integration/gaia-central/rev/a138fd9f4470
Author: Ghislain 'Aus' Lacroix <aus@mozilla.com>
Desc: bug 999511

* Do not use gecko provided title if it contains the origin of the URL of the application.

r=alive

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https://hg.mozilla.org/integration/gaia-central/rev/99f2208b05c3
Author: Staś Małolepszy <stas@mozilla.com>
Desc: Merge pull request #18746 from stasm/1001453-wappush-once

Bug 1001453 - Clean up bootstrap mozL10n API use in Wappush. r=gsvelto

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https://hg.mozilla.org/integration/gaia-central/rev/e7da444b4ea9
Author: Staś Małolepszy <stas@mozilla.com>
Desc: Bug 1001453 - Clean up bootstrap mozL10n API use in Wappush. r=gsvelto

b2g/config/gaia.json

@@ -4,6 +4,6 @@
         "remote": "", 
         "branch": ""
     }, 
-    "revision": "57cb6d635bad22cfab52e55c9987cfd8370fc8cd", 
+    "revision": "e151404830232e385c41d5fce72277c0e719419a", 
     "repo_path": "/integration/gaia-central"
 }