Bumping gaia.json for 3 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/8bb0cf53956e
Author: Mason Chang <mchang@mozilla.com>
Desc: Revert "Merge pull request #16647 from changm/bug975811"

This reverts commit 89950a14ec912d78f44345bd143fab1e0da23804, reversing
changes made to ff53e9a2cf3fb0d65b37831fb5bab6286a9f5d9a.

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https://hg.mozilla.org/integration/gaia-central/rev/7f00444fe49c
Author: Anthony Ricaud <anthony@ricaud.me>
Desc: Merge pull request #16656 from Rik/telephony-promise-969280

Bug 969280 - We should support old and new (promise) telephony API r=etienne

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https://hg.mozilla.org/integration/gaia-central/rev/822886ec78e8
Author: Anthony Ricaud <anthony@ricaud.me>
Desc: Bug 969280 - We should support old and new (promise) telephony API

b2g/config/gaia.json

@@ -4,6 +4,6 @@
         "branch": "", 
         "revision": ""
     }, 
-    "revision": "e88fea50cbf4bd2465ae9a17038274c57e61d90c", 
+    "revision": "8bb0cf53956e54999a5f876434207216d9d8982a", 
     "repo_path": "/integration/gaia-central"
 }