Enable heartbeat montoring on default

With the change, the heartbeat monitor will kill the engine after
50 missed heartbeats (~150 secs under the default config).

IPython/parallel/engine/engine.py

@@ -53,7 +53,7 b' class EngineFactory(RegistrationFactory):'
     timeout=Float(5.0, config=True,
         help="""The time (in seconds) to wait for the Controller to respond
         to registration requests before giving up.""")
-    max_heartbeat_misses=Integer(0, config=True,
+    max_heartbeat_misses=Integer(50, config=True,
         help="""The maximum number of times a check for the heartbeat ping of a 
         controller can be missed before shutting down the engine.