MdnsListener has no attribute remove_service
AttributeError when using zeroconf mDNS.

```
 * Running test_13
Exception in thread zeroconf-ServiceBrowser__nmos-node._tcp.local.:
Traceback (most recent call last):
  File "/usr/lib/python3.5/threading.py", line 914, in _bootstrap_inner
    self.run()
  File "/usr/local/lib/python3.5/dist-packages/zeroconf.py", line 1382, in run
    handler(self.zc)
  File "/usr/local/lib/python3.5/dist-packages/zeroconf.py", line 1329, in <lambda>
    state_change=state_change,
  File "/usr/local/lib/python3.5/dist-packages/zeroconf.py", line 1235, in fire
    h(**kwargs)
  File "/usr/local/lib/python3.5/dist-packages/zeroconf.py", line 1304, in on_change
    listener.remove_service(*args)
AttributeError: 'MdnsListener' object has no attribute 'remove_service'

```
diff --git a/MdnsListener.py b/MdnsListener.py
index b0627a7..c977494 100644
--- a/MdnsListener.py
+++ b/MdnsListener.py
@@ -30,6 +30,9 @@ class MdnsListener(object):
         t.daemon = True
         t.start()
 
+    def remove_service(self, zeroconf, srv_type, name):
+        pass
+
     def get_service_list(self):
         self.resolve_queue.join()
         return self.services
['MdnsListener.py']