Fix ipykernel install

[davidbrochart]

Closes #1309

[krassowski]

This works, thank you!

Do I guess right that the issue was introduced in #1079 by this change:

if self.subapp is not None:
+    self.subapp.start()
-    return self.subapp.start()

I wonder if return status here makes a difference for some command line applications.

[davidbrochart]

if self.subapp is not None:
+    self.subapp.start()
-    return self.subapp.start()

Actually I think we should rather revert that change, thanks for finding it.