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

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https://hg.mozilla.org/integration/gaia-central/rev/a51e060e805e
Author: Jose M. Cantera <jmcanterafonseca@gmail.com>
Desc: Merge pull request #13663 from gtorodelvalle/contacts-bug-937040-removing-and-saving-mozContacts

Bug 937040 - Revise contacts app to adapt to WebIDL changes in a uniform way (r=jmcanterafonseca)

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https://hg.mozilla.org/integration/gaia-central/rev/ac152fd998b9
Author: Germán Toro del Valle <gtorodelvalle@gmail.com>
Desc: Bug 937040 - Revise contacts app to adapt to WebIDL changes in a uniform way

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https://hg.mozilla.org/integration/gaia-central/rev/9c938127718e
Author: Zac <zcampbell@mozilla.com>
Desc: Merge pull request #13692 from qba73/emailkeyboard

Bug 938577 - Adding email keyboard test.

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https://hg.mozilla.org/integration/gaia-central/rev/0f1308512f94
Author: Jakub Jarosz <jakub.s.jarosz@gmail.com>
Desc: Adding email keyboard test.

b2g/config/gaia.json

@@ -1,4 +1,4 @@
 {
-    "revision": "1607c58d20be15372cbee096059ddd39e08da601", 
+    "revision": "a51e060e805e8b8aa802a361469b0a440a5713aa", 
     "repo_path": "/integration/gaia-central"
 }