Without wget, Vagrant tries to download Discourse using some other way than wget (curl, perhaps?), and that for some reason doesn’t work (on Arch Linux, I wonder if it’s any different on other operating systems). Without it, “504 Gateway Timeout” happens. The solution is to install wget. Putting this thread, because this issue confused me for a while, and it magically solved itself when I installed wget - figured it may be useful for others.
~/K/F/discourse (master|✔) $ vagrant up
Bringing machine 'default' up with 'virtualbox' provider...
==> default: Box 'discourse-16.04' could not be found. Attempting to find and install...
default: Box Provider: virtualbox
default: Box Version: >= 0
==> default: Box file was not detected as metadata. Adding it directly...
==> default: Adding box 'discourse-16.04' (v0) for provider: virtualbox
default: Downloading: https://www.dropbox.com/s/2132770g1e05c6d/discourse.box?dl=1
An error occurred while downloading the remote file. The error
message, if any, is reproduced below. Please fix this error and try
again.
The requested URL returned error: 504 Gateway Timeout
~/K/F/discourse (master|✔) $ vagrant --version
Vagrant 1.9.4