Fix missing brackets in speaker.py testing code

Author: Holzhaus

client/speaker.py

@@ -242,7 +242,7 @@ def newSpeaker():
     for engine in AbstractSpeaker.__subclasses__():
         if hasattr(engine, 'SLUG'):
             instance = engine()
-            if instance.is_available:
+            if instance.is_available():
                 engines.append(instance)
 
     for engine in engines: