Solution taken from http://superuser.com/a/180149 - IMPORTANT: chmod 755 ~/.ssh/rc

.ssh-rc

#!/bin/bash
if test "$SSH_AUTH_SOCK" ; then
    ln -sf $SSH_AUTH_SOCK ~/.ssh/ssh_auth_sock
fi

if read proto cookie && [ -n "$DISPLAY" ]; then
    if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
        # X11UseLocalhost=yes
        echo add unix:`echo $DISPLAY |
            cut -c11-` $proto $cookie
    else
        # X11UseLocalhost=no
        echo add $DISPLAY $proto $cookie
    fi | xauth -q -
fi