Compatibility with Ruby 2.7

update arg count

priority_queue.c: In function ‘pq_node2dot’:
/usr/local/include/ruby-2.7.0/ruby/ruby.h:1871:3: error: call to ‘rb_varargs_bad_length’ declared with attribute error: 
argument length doesn't match
   rb_varargs_bad_length(argc, vargc)), \
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/local/include/ruby-2.7.0/ruby/ruby.h:2690:6: note: in expansion of macro ‘rb_varargs_argc_check’
      rb_varargs_argc_check(rb_funcall_argc, rb_funcall_nargs), \
      ^~~~~~~~~~~~~~~~~~~~~
priority_queue.c:820:4: note: in expansion of macro ‘rb_funcall’
    rb_funcall(Qnil, id_format, 4, rb_str_new2("NODE%i -> NODE%i;\n"),
    ^~~~~~~~~~
make: *** [Makefile:245: priority_queue.o] Error 1