Investigating the graphical display configuration tools I was unable to find an option to change this. Luckily xrandr supports the setting of the primary display and GNOME 3 appears to take heed of it. To change primary display:
- Run xrandr to list available displays.
- Run xrandr --output <display> --primary to set the primary display.
Update: Another viable solution has been posted in the comments. Unlike the one I have posted above it does not need to be done every time you log in. I am not sure how it performs in the event that you disconnect/reconnect the monitor or dock/undock the laptop frequently.