Add Citrix Receiver

I left the commented uninstall line as a point of discussion being my first contribution. Citrix provides an Uninstall app in the download, but I couldn't figure out how to make use of it w/ Cask.  I ended up using pkgutil to do the removal, and it seems to work.  If you're happy with using the pkgutil uninstall over the .app, I'm happy to remove the commented line.