Fixed uninstall for GoogleHangouts cask

I kept getting permission problems with the original pkgutil uninstall method. In order to get this to work, I called '/usr/bin/open' and passed the uninstaller application path as an arg. I'm not sure I did the right thing, but didn't see a good way to do this otherwise. If there is a better way to pass an application to uninstall, please reply and reject this pull request and I will fix.