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

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https://hg.mozilla.org/integration/gaia-central/rev/af2c025bbb50
Author: Jose M. Cantera <jmcanterafonseca@gmail.com>
Desc: Merge pull request #12431 from jmcanterafonseca/bug-918674

Bug 918674 - In Contacts header the separator between '+' and 'settings ...

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https://hg.mozilla.org/integration/gaia-central/rev/3238bfb629bf
Author: Jose M. Cantera <jmcanterafonseca@gmail.com>
Desc: Bug 918674 - In Contacts header the separator between '+' and 'settings icon' has disappeared

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https://hg.mozilla.org/integration/gaia-central/rev/9fdcb2d625f1
Author: gasolin <gasolin@gmail.com>
Desc: Merge pull request #12359 from gasolin/issue-919455

Bug 919455 - tablet wallpaper are stretched, r=yurenju

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https://hg.mozilla.org/integration/gaia-central/rev/05ecb8099fd7
Author: gasolin <gasolin@gmail.com>
Desc: Bug 919455 - tablet wallpaper are stretched

b2g/config/gaia.json

@@ -1,4 +1,4 @@
 {
-    "revision": "fd17e16521123febdbcb0946027b5a6fc1e6d0e0", 
+    "revision": "af2c025bbb504880ef58f777ee26cc23a250f299", 
     "repo_path": "/integration/gaia-central"
 }