Use default_popup_response=True in ghost.open

  * Using default_popup_response=True in ghost.open() calls appears to click through any popups or prompts that may come about, useful for invalid certs or JS popups that may arise and cause timeouts otherwise.
  * Feature details in ghost (best I could find): https://github.com/jeanphix/Ghost.py/commit/8522c532a39bbc2f73799afe15269e0d426bbd0d

This allowed me to successfully get screenshots of some sites that had self-signed SSL certs in use. *Maybe* this should be a configurable option in the future.