Bumping gaia.json for 2 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/8c153b0618e4
Author: Pavel Ivanov <pivanov@mozilla.com>
Desc: Merge pull request #15619 from pivanov/bug-935095

Bug 935095 - [Gallery] 1.3 Back button position not to spec

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https://hg.mozilla.org/integration/gaia-central/rev/746bc70f1a51
Author: Pavel Ivanov <pivanov@mozilla.com>
Desc: Bug 935095 - [Gallery] 1.3 Back button position not to spec

b2g/config/gaia.json

@@ -1,4 +1,4 @@
 {
-    "revision": "b7c6efd07444c2d0602bf734e618a9ace357be56", 
+    "revision": "8c153b0618e4dcd9c6798994b288b880d5dfa405", 
     "repo_path": "/integration/gaia-central"
 }