AppVeyor - Fix freetds install errors by updating packages

[bvogelzang]

AppVeyor builds have been failing during freetds installation due to outdated packages:

warning: no /var/cache/pacman/pkg/ cache exists, creating...
error: failed retrieving file 'mingw-w64-i686-freetds-1.00.100-1-any.pkg.tar.xz' from repo.msys2.org : The requested URL returned error: 404
error: failed retrieving file 'mingw-w64-i686-freetds-1.00.100-1-any.pkg.tar.xz' from sourceforge.net : The requested URL returned error: 404
error: failed retrieving file 'mingw-w64-i686-freetds-1.00.100-1-any.pkg.tar.xz' from www2.futureware.at : The requested URL returned error: 404
error: failed retrieving file 'mingw-w64-i686-freetds-1.00.100-1-any.pkg.tar.xz' from mirror.yandex.ru : The requested URL returned error: 404
warning: failed to retrieve some files
error: failed to commit transaction (unexpected error)

Adds install commands to the appveyor.yml so that freetds installs successfully.

I would expect running something similar to pacman -Syu would do the job, however, it appears there were recent changes to MSYS2 that require more setup. These commands were borrowed from https://github.com/oneclick/rubyinstaller2/blob/566169328a88c4960c75f0b25ef0c6fb5327fb46/appveyor.yml#L14-L31

[aharpervc]

LGTM

[aharpervc]

Can you rebase your other two PRs on this branch to confirm that with these changes, they pass CI?