Remove dependency to obsolete IO::Socket::INET6
    
    In scripts/mosh.pl is code:
    
    my $have_ipv6 = eval {
      require IO::Socket::IP;
      IO::Socket::IP->import('-register');
      1;
    } || eval {
      require IO::Socket::INET6;
      1;
    };
    
    This mean, that we could remove usage IO::Socket::INET6 and add
    IO::Socket::IP to Requires.