Update virtualbox - add uninstall preflight

[commitay]

After making all changes to the cask:

• brew cask audit --download {{cask_file}} is error-free.
• brew cask style --fix {{cask_file}} reports no offenses.
• The commit message includes the cask’s name and version.

Checks to see if virtualbox-extension-pack exists at "#{HOMEBREW_PREFIX}/Caskroom/virtualbox-extension-pack" and uninstalls if it is found.

#29301 (comment)
virtualbox was updated today with a minor version change which happens to be a dependent of virtualbox-extension-pack. Reinstall works fine, but then the extension pack is missing because everything had been deleted even though cask shows the package as installed.

I've had this happen a few times with updates or switching between virtualbox / virtualbox-beta, also you can't uninstall virtualbox-extension-pack without virtualbox.

/cc @colindunn

[colindunn]

Makes sense to me but it would mean that you have to install the extension pack again as a slight gotya. Seems like a reasonable trade-off.