Update digest for vim@8.0 tarball. (#2600)

* Update digest for vim@8.0 tarball.

The digest appears to have changed.

The new digest value matches the value in their
[MD5SUMS](ftp://ftp.vim.org/pub/vim/unix/MD5SUMS)
file.

* Change ftp.vim.org -> github.com/vim/vim/archive

Vim seems to do weekly releases, but the name of the tarball on
ftp.vim.org doesn't change so we have to regularly play 'update the
digest' (aka Internet whack-a-mole).

This commit changes the url so that we are now downloading particular
versions from the Vim project's github archive.

I didn't walk back through of time to get all of the versions that
used to be explicit.  I grabbed the final `7.4` and the current `8.0`
releases.  If people need more we can add them.