1. make sure .ssh/ folder has the write permissions 700:
chmod 700 .ssh
2. the file .ssh/authorized_keys should have the write permissions 600:
chmod 600 .ssh/authorized_keys
3. in the file /etc/ssh/sshd_config the line PermitRootLogin should be like this:
PermitRootLogin without-password
more details you can find here: https://www.digitalocean.com/community/tutorials/initial-server-setup-with-ubuntu-14-04