Fetch zips from github for DMD 2.057+

[Abscissa]

From 2.057 onward, DMD's zips are available on GitHub, which is much much faster to download from:

https://github.com/D-Programming-Language/dmd/downloads

When fetching DMD 2.057 or newer, DVM should fetch from github instead of the much slower ftp.digitalmars.com.

[jacob-carlborg]

Fixed in e66951c.