mark Ubuntu xenial as supported

mmoll · mmoll · commit 05ba436b0abb · 2016-02-07T02:05:28.000+01:00
diff --git a/metadata.json b/metadata.json
@@ -63,7 +63,8 @@
       "operatingsystemrelease": [
         "10.04",
         "12.04",
-        "14.04"
+        "14.04",
+        "16.04"
       ]
     }
   ],
