Add cask for VisTrails 2.0.3.
user
```ruby
class Vistrails < Cask
  url 'http://downloads.sourceforge.net/project/vistrails/vistrails/v2.0.3/vistrails-mac-10.6-intel-2.0.3-a6ad79347667.dmg'
  homepage 'http://www.vistrails.org/index.php/Main_Page'
  version '2.0.3'
  sha1 '1381f58a8946792ed448ae072607cee9ed32a041'
  link 'VisTrails'
end
```
assistant