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

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https://hg.mozilla.org/integration/gaia-central/rev/7b310a78ae68
Author: Dominic Kuo <dkuo@mozilla.com>
Desc: Merge pull request #19198 from dominickuo/bug-998911

Bug 998911 - [Settings] Visual update for the volume sliders in sound region, r=crh0716

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https://hg.mozilla.org/integration/gaia-central/rev/a6fd232929a2
Author: Dominic Kuo <dkuo@mozilla.com>
Desc: Bug 998911 - [Settings] Visual update for the volume sliders in sound region

b2g/config/gaia.json

@@ -4,6 +4,6 @@
         "remote": "", 
         "branch": ""
     }, 
-    "revision": "eff498cf19a780a9e555b50221c54a831b5405cd", 
+    "revision": "7b310a78ae6876ab7b80025fc13054c502276b83", 
     "repo_path": "/integration/gaia-central"
 }