That's strange that Arduino AVR Boards 1.6.20 didn't work for you but I'm just glad to hear you managed to get it working. The problems you were having were caused by using a very outdated version of Arduino AVR Boards, which you probably still had from a previous version of the Arduino IDE. The current version of Arduino AVR Boards is actually included with your Arduino IDE installation but if you have previously updated to a different Arduino AVR Boards version that version will be used instead.
Regarding the Ethernet2 warning, that indicates you're using an old version of the library. You should be able to solve that by doing this:
- Sketch > Include Library > Manage Libraries
- Wait for download to finish
- In the "Filter your search..." box, type "ethernet2"
- Click on the Ethernet2 entry
- Click "Update"
- Wait for update to finish
- Click "Close"