Fix KeyError: 'suppress_ignore_reports'

[...]
  File "/home/archivebot/.local/lib/python3.4/site-packages/wpull/hook.py", line 433, in _handle_pre_response
    url_info_dict, url_record_dict, response_info_dict
  File "wpull_hooks.py", line 216, in handle_pre_response
    maybe_log_ignore(url, '[icy field]')
  File "wpull_hooks.py", line 42, in maybe_log_ignore
    if not settings.suppress_ignore_reports():
  File "pipeline/archivebot/wpull/settings.py", line 94, in suppress_ignore_reports
    return self.settings['suppress_ignore_reports']
KeyError: 'suppress_ignore_reports'