I get a 404, not 403 but it's certainly an issue either way. I have already reported this to the Arduino developers earlier today at http://downloads.arduino.cc/tools/avrdude-6.3.0-arduino6-i686-mingw32.zip is missing · Issue #5400 · arduino/Arduino · GitHub. I'm suer they'll have it fixed in a few hours. Until then you can just download Arduino IDE 1.6.12 from http://www.arduino.cc/en/Main/Software, which includes avrdude 6.3.0-arduino6 at arduino-1.6.12\hardware\tools\avr\bin\avrdude.exe(I think there's a .dll file in that folder that may be necessary also), and C:\Program Files (x86)\arduino-1.6.12\hardware\tools\avr\etc\avrdude.conf.