I also have the same problem.
The fix with the command line didn't help.
Could it be, that the Github boardpackage download is down?
When I open: http://arduino.esp8266.com/versions/2.3 ... index.json
and try to download manually the .zip package that is linked in the json, I get a connection timeout.
The URL inside the .json: https://github.com/esp8266/Arduino/rele ... -2.3.0.zip
Has anyone please help me?