use regular macros instead of compatible macros

According to mruby/include/mruby.h,

```c
/* compatibility macros; will be removed */
#define ARGS_REQ(n)         MRB_ARGS_REQ(n)
#define ARGS_OPT(n)         MRB_ARGS_OPT(n)
#define ARGS_REST()         MRB_ARGS_REST()
#define ARGS_POST(n)        MRB_ARGS_POST()
#define ARGS_KEY(n1,n2)     MRB_ARGS_KEY(n1,n2)
#define ARGS_BLOCK()        MRB_ARGS_BLOCK()
#define ARGS_ANY()          MRB_ARGS_ANY()
#define ARGS_NONE()         MRB_ARGS_NONE()
```