Ruby bug #9344 - onig_verb_warn doesn't respect instance_eval's file and line

This fixes #31 and Ruby bug [#9344](https://bugs.ruby-lang.org/issues/9344)

This fix is simply just to not use `onig_verb_warn`; it doesn't respect `instance_eval` sourcefile and sourceline, and `onig_syntax_warn` is a much cleaner API anyway.