Use `NIL_P` instead of `RB_NIL_P`

`RB_NIL_P` is new in Ruby 2.4. Even though we're linking to the older
name, I think it still makes sense to expose the newer name to Rust.

ruby/ext/helix_runtime/native/helix_runtime.c

@@ -45,7 +45,7 @@ bool HELIX_RB_TYPE_P(VALUE v, int type) {
 }
 
 bool HELIX_RB_NIL_P(VALUE v) {
-  return RB_NIL_P(v);
+  return NIL_P(v);
 }
 
 bool HELIX_RTEST(VALUE v) {