boards/arduino-due: force bossa 1.8 version
boards/arduino-due: force bossa 1.8 version

kYc0o/RIOT,miri64/RIOT,miri64/RIOT,RIOT-OS/RIOT,ant9000/RIOT,RIOT-OS/RIOT,yogo1212/RIOT,OTAkeys/RIOT,basilfx/RIOT,smlng/RIOT,kaspar030/RIOT,kYc0o/RIOT,authmillenon/RIOT,miri64/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,kaspar030/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,yogo1212/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,authmillenon/RIOT,smlng/RIOT,jasonatran/RIOT,jasonatran/RIOT,smlng/RIOT,yogo1212/RIOT,ant9000/RIOT,OlegHahm/RIOT,yogo1212/RIOT,smlng/RIOT,authmillenon/RIOT,kaspar030/RIOT,jasonatran/RIOT,kaspar030/RIOT,basilfx/RIOT,kYc0o/RIOT,ant9000/RIOT,ant9000/RIOT,yogo1212/RIOT,kYc0o/RIOT,OTAkeys/RIOT,jasonatran/RIOT,miri64/RIOT,yogo1212/RIOT,basilfx/RIOT,ant9000/RIOT,kYc0o/RIOT,authmillenon/RIOT,basilfx/RIOT,smlng/RIOT,miri64/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,basilfx/RIOT,authmillenon/RIOT,OlegHahm/RIOT