Add `brew update` to CONTRIBUTING file.
zsh 5.0.4

Closes #25376.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>
libsass 1.0.1

Closes #25362.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>
sassc 1.0.1

Closes #25363.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>
dpkg 1.17.5

The 1.17.1 tarball was no longer available from the upstream FTP.

Closes #25384.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>