Stop using `appveyor` command

[Thomas Johansen]

The use of the appveyor command in the PowerShell script is currently preventing the CI script from being executed in other environments.

There is no good reason for using that command, so we should replace it.

[Landon Abney]

AppVeyor lists several alternatives here: https://www.appveyor.com/docs/how-to/download-file/

[Joe Fitzgerald]

I reverted this in 3ff137a, per reasoning in #55.