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

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https://hg.mozilla.org/integration/gaia-central/rev/a4cf8ffc89da
Author: lissyx <lissyx+github@lissyx.dyndns.org>
Desc: Merge pull request #13455 from lissyx/bug935832

Bug 935832 - Add some integration testing on SIM customization r=lightsofapollo,aus,arthurcc

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https://hg.mozilla.org/integration/gaia-central/rev/a67cd1b25359
Author: Alexandre Lissy <lissyx+github@lissyx.dyndns.org>
Desc: Bug 935832 - Add some integration testing on SIM customization

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https://hg.mozilla.org/integration/gaia-central/rev/b579ac38ed8c
Author: Germán Toro del Valle <gtorodelvalle@gmail.com>
Desc: Merge pull request #13585 from gtorodelvalle/contacts-bug-924240-contact-deletion-from-ui

Bug 924240 - [B2G][Email][Contacts] Deleted contact is not erased from UI

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https://hg.mozilla.org/integration/gaia-central/rev/913dd2e788a6
Author: Germán Toro del Valle <gtorodelvalle@gmail.com>
Desc: Bug 924240 - [B2G][Email][Contacts] Deleted contact is not erased from UI

b2g/config/gaia.json

@@ -1,4 +1,4 @@
 {
-    "revision": "293d90fe0678cef411bc3f647304921f377249b3", 
+    "revision": "a4cf8ffc89dac77d44643fe527fd420f76ad663b", 
     "repo_path": "/integration/gaia-central"
 }