use get_shell_msg helper method (#368)

* use get_shell_msg helper method

Author: tschaume

kernel_gateway/services/kernels/manager.py

@@ -101,7 +101,7 @@ def start_kernel(self, *args, **kwargs):
                     # executed
                     if self.parent.personality.should_seed_cell(code):
                         client.execute(code)
-                        msg = client.shell_channel.get_msg(block=True)
+                        msg = client.get_shell_msg()
                         if msg['content']['status'] != 'ok':
                             # Shutdown the channels to remove any lingering ZMQ messages
                             client.stop_channels()