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

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https://hg.mozilla.org/integration/gaia-central/rev/fd971998ab41
Author: Zac <zcampbell@mozilla.com>
Desc: Merge pull request #21927 from zacc/bug_1039966

Bug 1039966 - Update gaiatest marionette_client to 0.8 and dependency mo...

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https://hg.mozilla.org/integration/gaia-central/rev/db13290b1ad8
Author: Zac <zcampbell@mozilla.com>
Desc: Bug 1039966 - Update gaiatest marionette_client to 0.8.1 and dependency moztest to >= 0.6

b2g/config/gaia.json

@@ -4,6 +4,6 @@
         "remote": "", 
         "branch": ""
     }, 
-    "revision": "494dce9a5bbb1baed5fcb5a57d4aed28cbfdf85f", 
+    "revision": "fd971998ab41e56d091651de74a12c889a8aafa3", 
     "repo_path": "/integration/gaia-central"
 }