Here’s a workaround I came up with:
First, download the box manually:
wget -N -O /opt/discourse.box https://www.dropbox.com/s/2132770g1e05c6d/discourse.box?dl=1
Then, edit Vagrantfile:
# Replace
conf.vm.box_url = "https://www.dropbox.com/s/2132770g1e05c6d/discourse.box?dl=1"
# with
conf.vm.box_url = "file:///opt/discourse.box"
Now, vagrant up will use the downloaded box.
Note: You have to re-download the image from time to time. The -N option signals wget to only download the box if it has changed.