Fix for snapshotting bug involving runscript

The runscript class was causing snapshotting problems due to two
of its parameters (ns and push) being set to unpicklable IPython
objects. Setting the pickle_default_value flag to False prevents
these parameter values from being included in the snapshot and
fixes the acute bug. Should consider adding an error catching
mechanism to PicklableClassAttributes to warn if an object is
unpicklable.