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

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https://hg.mozilla.org/integration/gaia-central/rev/644d444e2368
Author: Francisco Jordano <arcturus@ardeenelinfierno.com>
Desc: Merge pull request #13027 from jmcanterafonseca/fix_mozContact_merge

Bug 929914 - Revise contacts merging with respect to mozContact construc...

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https://hg.mozilla.org/integration/gaia-central/rev/78ee62c38fb2
Author: Jose M. Cantera <jmcanterafonseca@gmail.com>
Desc: Bug 929914 - Revise contacts merging with respect to mozContact constructor

b2g/config/gaia.json

@@ -1,4 +1,4 @@
 {
-    "revision": "2e293f22489ffe63fdf54656cefe8060eff33be7", 
+    "revision": "644d444e236827774ad6f6ecacf46bc55a6834fe", 
     "repo_path": "/integration/gaia-central"
 }