Add cask for VisIt 2.6.3.

Add cask for VisIt, an interactive visualization and graphical analysis
tool used for viewing and analyzing scientific data.

The latest version is 2.7.0, however, downloading and installing it
triggered a virus warning on my computer, and the binary executable
will not run. I am in the process of reporting this issue upstream.
Version 2.6.3 downloads cleanly, installs, and runs on my system,
so the cask downloads that version instead.