Download installer from Github repository

[webcoyote]

Hello. I've created this PR to download the Growl installer from Brian Dunnington's Github repository instead of the www.growlforwindows.com, as the installer is no longer available.

I've tested this locally and it seems to work fine. Please let me know if you'd like me to make changes, otherwise can I request that you upload this to Chocolatey, or add me as a maintainer so I can? If you'd like to make me a maintainer, my handle on Chocolatey is webcoyote, same as for Github.

Thanks,

Pat