Add virtualbox v4.3.20

There is an [issue](https://www.virtualbox.org/ticket/13874) with the current (as of 2/20/2015) version of
VirtualBox.

Adding this to allow installation of the last working version.

Casks/virtualbox432096996.rb

@@ -0,0 +1,11 @@
+cask :v1 => 'virtualbox432096996' do
+  version '4.3.20-96996'
+  sha256 '744e77119a640a5974160213c9912568a3d88dbd06a2fc6b6970070941732705'
+
+  url "http://download.virtualbox.org/virtualbox/4.3.20/VirtualBox-#{version}-OSX.dmg"
+  homepage 'http://www.virtualbox.org'
+  license :unknown
+
+  pkg 'VirtualBox.pkg'
+  uninstall :script => { :executable => 'VirtualBox_Uninstall.tool', :args => %w[--unattended] }
+end