retrieve download link using regex, more suitable package icon

* Download link for installer/zip file gets retrieved using regex, which will always download the latest version. This fixes all the 404 Errors and makes your package reliable and working all the time.
* Used the official desktop icon for imagemagick to comply with the new chocolatey package icon guidelines: https://github.com/chocolatey/chocolatey/wiki/CreatePackages#package-icon-guidelines