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

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https://hg.mozilla.org/integration/gaia-central/rev/ad813de39146
Author: Evyatar Amitay <evyatron@gmail.com>
Desc: Merge pull request #12528 from EverythingMe/grid-text-shadow

[Bug 912518] Increase grid icons' text shadow [r=crdlc]

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https://hg.mozilla.org/integration/gaia-central/rev/44b9089485ba
Author: Evyatar Amitay <evyatron@gmail.com>
Desc: [Bug 912518] Increase grid icons' text shadow [r=crdlc]

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https://hg.mozilla.org/integration/gaia-central/rev/8c46e41860c6
Author: Cristian Rodriguez <crdlc@tid.es>
Desc: Merge pull request #12434 from crdlc/bug-912520

Bug 912520 - New Underlay is needed

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https://hg.mozilla.org/integration/gaia-central/rev/d2937e0cc675
Author: crdlc <crdlc@tid.es>
Desc: Bug 912520 - New Underlay is needed

b2g/config/gaia.json

@@ -1,4 +1,4 @@
 {
-    "revision": "38aeb2b8ee6e3246dae7b3be02071acf52575e82", 
+    "revision": "ad813de39146f7a54d5c61a214f65653a17b01e0", 
     "repo_path": "/integration/gaia-central"
 }