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

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https://hg.mozilla.org/integration/gaia-central/rev/29e07fa56886
Author: Etienne Segonzac <etienne@segonzac.info>
Desc: Merge pull request #12081 from lissyx/bug910267

Bug 910267 - Add a busy tone when the callee is busy r=etienne

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https://hg.mozilla.org/integration/gaia-central/rev/728e07827392
Author: Alexandre Lissy <lissyx+github@lissyx.dyndns.org>
Desc: Bug 910267 - Add a busy tone when the callee is busy

Per cerfication requests we need to play a busy tone, not only display
the information to the user in a message. This commit adds a busy tone
playback. It also fixes the telephony helper tests.

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https://hg.mozilla.org/integration/gaia-central/rev/fc2af425d76a
Author: Francisco Jordano <arcturus@ardeenelinfierno.com>
Desc: Merge pull request #12048 from arcturus/bug-914130

Bug 914130 - Exporting contacts keeps the previous state

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https://hg.mozilla.org/integration/gaia-central/rev/6070ccbdde79
Author: Francisco Jordano <arcturus@ardeenelinfierno.com>
Desc: Bug 914130 - Exporting contacts keeps the previous state

b2g/config/gaia.json

@@ -1,4 +1,4 @@
 {
-    "revision": "6321667311ff6c8f2d718dd21d95733295fb721e", 
+    "revision": "29e07fa568868135b9091312abca4bfb2bb22610", 
     "repo_path": "/integration/gaia-central"
 }