add mirror url for avrdude

Hi guys! :)

The main [url](http://download.savannah.gnu.org/releases/avrdude/?C=M;O=D) is currently not workind.

I just added a [mirror](http://download-mirror.savannah.gnu.org/releases/avrdude/?C=M;O=D) to be able to download and install `avrdude` properly.

It works  nice now.