[dock] Remove decode option when reading stats from container

It was already not used when stream is set to false however in
more recent versions of the python docker bindings, an error is
raised if stream is set to false and and decode to true.

beat/core/dock.py

@@ -482,7 +482,7 @@ class Host(object):
 
         client = docker.APIClient()
 
-        data = client.stats(container.id, decode=True, stream=False)
+        data = client.stats(container.id, stream=False)
 
         # If CPU statistics can't be retrieved
         if 'system_cpu_usage' not in data['cpu_stats']: