Fix IPython.start_kernel()

Closes gh-4005

IPython/kernel/zmq/kernelapp.py

@@ -458,6 +458,7 b' class IPKernelApp(BaseIPythonApplication, InteractiveShellApp):'
         except KeyboardInterrupt:
             pass
 
+launch_new_instance = IPKernelApp.launch_instance
 
 def main():
     """Run an IPKernel as an application"""