Use MRB_ARGS_REST() instead of ARGS_REST()

According to include/mruby.h,

 /* compatibility macros; will be removed */
 #define ARGS_REST()         MRB_ARGS_REST()
Merge pull request #1921 from cubicdaiya/issues/not_use_old_macro

Use MRB_ARGS_REST() instead of ARGS_REST()