Be less restrictive on netrc permissions

My `.netrc` permission bits are `0400`, which shouldn't be a problem.  However, the test below fails.  It seems to be too restrictive.

    def self.check_permissions(path)
        perm = File.stat(path).mode & 0777
        if perm != 0600 && !(WINDOWS) && !(Netrc.config[:allow_permissive_netrc_file])
          raise Error, "Permission bits for '#{path}' should be 0600, but are "+perm.to_s(8)
        end
      end

All that's really required is `.netrc` is not world/group accessible.