Added call to decode data from Consul

On Python 3, consul call to `kv.get` returns a bytes type, calling
`.decode()` will turn that into a str object

Author: Rafael Sierra

decouple.py

@@ -137,7 +137,7 @@ def __contains__(self, key):
         if not data:
             return False
         else:
-            self.data[key] = data.get('Value', None)
+            self.data[key] = data.get('Value', None).decode()
             return True
 
     def __getitem__(self, key):