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

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https://hg.mozilla.org/integration/gaia-central/rev/18e2adbabcfa
Author: Hubert Figuière <hub@figuiere.net>
Desc: Merge pull request #15476 from hfiguiere/bug917717-2

Bug 917717 - Part 2: use the prebuilt ffi. r=gaye

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https://hg.mozilla.org/integration/gaia-central/rev/d045eca007b0
Author: Hubert Figuière <hub@figuiere.net>
Desc: Bug 917717 - Part 2: use the prebuilt ffi.

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https://hg.mozilla.org/integration/gaia-central/rev/333969d91595
Author: steveck-chung <schung@mozilla.com>
Desc: Merge pull request #15526 from steveck-chung/bug-961231

Bug 961231 - [B2G][SMS]To: Text field’s subtext region is truncated. r=julienw

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https://hg.mozilla.org/integration/gaia-central/rev/087bc9758258
Author: Steve Chung <schung@mozilla.com>
Desc: Bug 961231 - [B2G][SMS]To: Text field’s subtext region is truncated

b2g/config/gaia.json

@@ -1,4 +1,4 @@
 {
-    "revision": "a6ece290b97430be9fafb4a53bafe24f2fe5f0b4", 
+    "revision": "18e2adbabcfa967611077df5a9bff7f855041324", 
     "repo_path": "/integration/gaia-central"
 }