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

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https://hg.mozilla.org/integration/gaia-central/rev/d75252e83e48
Author: Rudy Lu <me@rudy.lu>
Desc: Merge pull request #21101 from RudyLu/system/Bug1030058-remove_assets

Bug 1030058 - apps/system/style/themes/default/images/ui/time_pattern.pn...
r=pivanov

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https://hg.mozilla.org/integration/gaia-central/rev/e17f6c52cc14
Author: Rudy Lu <me@rudy.lu>
Desc: Bug 1030058 - apps/system/style/themes/default/images/ui/time_pattern.png should be
removed.

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https://hg.mozilla.org/integration/gaia-central/rev/7759d2755889
Author: steveck-chung <schung@mozilla.com>
Desc: Merge pull request #19991 from rnowm/1013296

Bug 1013296 - Compose. Change send button to an paper plane icon. r=steveck-chung

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https://hg.mozilla.org/integration/gaia-central/rev/b48b7d480d04
Author: rnowm <arnau@arnaumarch.com>
Desc: Bug 1013296 - Compose. Change send button to an paper plane icon

b2g/config/gaia.json

@@ -4,6 +4,6 @@
         "remote": "", 
         "branch": ""
     }, 
-    "revision": "ffb53607d2941324814de17e527531b7ae91df77", 
+    "revision": "d75252e83e48713c76466e299805ae2b35078e6a", 
     "repo_path": "/integration/gaia-central"
 }