Kernel/notebook mapping is removed when a kernel dies.

* Previously, when a kernel died due to an external cause, the
  notebook/kernel mapping was not removed, so the kernel would
  be resused even though it was dead.
* The heartbeat now properly removes the notebook/kernel mapping.

IPython/frontend/html/notebook/handlers.py

@@ -175,6 +175,7 @@ def stop_hb(self):
                 self.hb_stream.on_recv(None)
 
     def kernel_died(self):
+        self.application.kernel_manager.delete_mapping_for_kernel(self.kernel_id)
         self.write_message(
             {'header': {'msg_type': 'status'},
              'parent_header': {},