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
Please revert this line back. It won’t work like you’re expecting, ad we shouldn’t be changing it anyway. See [this comment on a similar issue](https://github.com/caskroom/homebrew-versions/pull/386#issuecomment-53410571).