arduino-beta: fixing the download URL

the download URL is not available, raising the following error:
    curl: (22) The requested URL returned error: 403
    Error: Download failed on Cask 'arduino-beta' with message:
    Download failed: http://downloads.arduino.cc/arduino-1.5.8-macosx-java7-r2.zip
This should be `url "http://arduino.cc/download.php?f=/arduino-#{version}-macosx-java7.zip"`. There’s no reason to lose the capabilities of `#{version}`.